Restored top-level documentation for ActionController::Base.

This information was lost in commit bd6b61b.
This might have been intentional, but this class does represent the starting
point for all things related to actions, and as such should document it.

I couldn't find any trace of this documentation, which seems like a waste.
Updated parts here and there to conform to current best practices.

actionpack/lib/action_controller/base.rb

@@ -1,6 +1,169 @@
 require "action_controller/log_subscriber"
 
 module ActionController
+  # Action Controllers are the core of a web request in \Rails. They are made up of one or more actions that are executed
+  # on request and then either render a template or redirect to another action. An action is defined as a public method
+  # on the controller, which will automatically be made accessible to the web-server through \Rails Routes.
+  #
+  # By default, only the ApplicationController in a \Rails application inherits from <tt>ActionController::Base</tt>. All other
+  # controllers in turn inherit from ApplicationController. This gives you one class to configure things such as
+  # request forgery protection and filtering of sensitive request parameters.
+  #
+  # A sample controller could look like this:
+  #
+  #   class PostsController < ApplicationController
+  #     def index
+  #       @posts = Post.all
+  #     end
+  #
+  #     def create
+  #       @post = Post.create params[:post]
+  #       redirect_to posts_path
+  #     end
+  #   end
+  #
+  # Actions, by default, render a template in the <tt>app/views</tt> directory corresponding to the name of the controller and action
+  # after executing code in the action. For example, the +index+ action of the PostsController would render the
+  # template <tt>app/views/posts/index.erb</tt> by default after populating the <tt>@posts</tt> instance variable.
+  #
+  # Unlike index, the create action will not render a template. After performing its main purpose (creating a
+  # new post), it initiates a redirect instead. This redirect works by returning an external
+  # "302 Moved" HTTP response that takes the user to the index action.
+  #
+  # These two methods represent the two basic action archetypes used in Action Controllers. Get-and-show and do-and-redirect.
+  # Most actions are variations of these themes.
+  #
+  # == Requests
+  #
+  # For every request, the router determines the value of the +controller+ and +action+ keys. These determine which controller
+  # and action are called. The remaining request parameters, the session (if one is available), and the full request with
+  # all the HTTP headers are made available to the action through accessor methods. Then the action is performed.
+  #
+  # The full request object is available via the request accessor and is primarily used to query for HTTP headers:
+  #
+  #   def server_ip
+  #     location = request.env["SERVER_ADDR"]
+  #     render :text => "This server hosted at #{location}"
+  #   end
+  #
+  # == Parameters
+  #
+  # All request parameters, whether they come from a GET or POST request, or from the URL, are available through the params method
+  # which returns a hash. For example, an action that was performed through <tt>/posts?category=All&limit=5</tt> will include
+  # <tt>{ "category" => "All", "limit" => 5 }</tt> in params.
+  #
+  # It's also possible to construct multi-dimensional parameter hashes by specifying keys using brackets, such as:
+  #
+  #   <input type="text" name="post[name]" value="david">
+  #   <input type="text" name="post[address]" value="hyacintvej">
+  #
+  # A request stemming from a form holding these inputs will include <tt>{ "post" => { "name" => "david", "address" => "hyacintvej" } }</tt>.
+  # If the address input had been named "post[address][street]", the params would have included
+  # <tt>{ "post" => { "address" => { "street" => "hyacintvej" } } }</tt>. There's no limit to the depth of the nesting.
+  #
+  # == Sessions
+  #
+  # Sessions allows you to store objects in between requests. This is useful for objects that are not yet ready to be persisted,
+  # such as a Signup object constructed in a multi-paged process, or objects that don't change much and are needed all the time, such
+  # as a User object for a system that requires login. The session should not be used, however, as a cache for objects where it's likely
+  # they could be changed unknowingly. It's usually too much work to keep it all synchronized -- something databases already excel at.
+  #
+  # You can place objects in the session by using the <tt>session</tt> method, which accesses a hash:
+  #
+  #   session[:person] = Person.authenticate(user_name, password)
+  #
+  # And retrieved again through the same hash:
+  #
+  #   Hello #{session[:person]}
+  #
+  # For removing objects from the session, you can either assign a single key to +nil+:
+  #
+  #   # removes :person from session
+  #   session[:person] = nil
+  #
+  # or you can remove the entire session with +reset_session+.
+  #
+  # Sessions are stored by default in a browser cookie that's cryptographically signed, but unencrypted.
+  # This prevents the user from tampering with the session but also allows him to see its contents.
+  #
+  # Do not put secret information in cookie-based sessions!
+  #
+  # Other options for session storage:
+  #
+  # * ActiveRecord::SessionStore - Sessions are stored in your database, which works better than PStore with multiple app servers and,
+  #   unlike CookieStore, hides your session contents from the user. To use ActiveRecord::SessionStore, set
+  #
+  #     config.action_controller.session_store = :active_record_store
+  #
+  #   in your <tt>config/environment.rb</tt> and run <tt>rake db:sessions:create</tt>.
+  #
+  # == Responses
+  #
+  # Each action results in a response, which holds the headers and document to be sent to the user's browser. The actual response
+  # object is generated automatically through the use of renders and redirects and requires no user intervention.
+  #
+  # == Renders
+  #
+  # Action Controller sends content to the user by using one of five rendering methods. The most versatile and common is the rendering
+  # of a template. Included in the Action Pack is the Action View, which enables rendering of ERb templates. It's automatically configured.
+  # The controller passes objects to the view by assigning instance variables:
+  #
+  #   def show
+  #     @post = Post.find(params[:id])
+  #   end
+  #
+  # Which are then automatically available to the view:
+  #
+  #   Title: <%= @post.title %>
+  #
+  # You don't have to rely on the automated rendering. Especially actions that could result in the rendering of different templates will use
+  # the manual rendering methods:
+  #
+  #   def search
+  #     @results = Search.find(params[:query])
+  #     case @results
+  #       when 0 then render :action => "no_results"
+  #       when 1 then render :action => "show"
+  #       when 2..10 then render :action => "show_many"
+  #     end
+  #   end
+  #
+  # Read more about writing ERb and Builder templates in ActionView::Base.
+  #
+  # == Redirects
+  #
+  # Redirects are used to move from one action to another. For example, after a <tt>create</tt> action, which stores a blog entry to a database,
+  # we might like to show the user the new entry. Because we're following good DRY principles (Don't Repeat Yourself), we're going to reuse (and redirect to)
+  # a <tt>show</tt> action that we'll assume has already been created. The code might look like this:
+  #
+  #   def create
+  #     @entry = Entry.new(params[:entry])
+  #     if @entry.save
+  #       # The entry was saved correctly, redirect to show
+  #       redirect_to :action => 'show', :id => @entry.id
+  #     else
+  #       # things didn't go so well, do something else
+  #     end
+  #   end
+  #
+  # In this case, after saving our new entry to the database, the user is redirected to the <tt>show</tt> method which is then executed.
+  #
+  # == Calling multiple redirects or renders
+  #
+  # An action may contain only a single render or a single redirect. Attempting to try to do either again will result in a DoubleRenderError:
+  #
+  #   def do_something
+  #     redirect_to :action => "elsewhere"
+  #     render :action => "overthere" # raises DoubleRenderError
+  #   end
+  #
+  # If you need to redirect on the condition of something, then be sure to add "and return" to halt execution.
+  #
+  #   def do_something
+  #     redirect_to(:action => "elsewhere") and return if monkeys.nil?
+  #     render :action => "overthere" # won't be called if monkeys is nil
+  #   end
+  #
   class Base < Metal
     abstract!