Add documentation

lib/rack/config.rb

@@ -1,6 +1,11 @@
 module Rack
   # Rack::Config modifies the environment using the block given during
   # initialization.
+  #
+  # Example:
+  #     use Rack::Config do |env|
+  #       env['my-key'] = 'some-value'
+  #     end
   class Config
     def initialize(app, &block)
       @app = app

lib/rack/head.rb

@@ -1,6 +1,8 @@
 module Rack
 
 class Head
+  # Rack::Head returns an empty body for all HEAD requests. It leaves
+  # all other requests unchanged.
   def initialize(app)
     @app = app
   end

lib/rack/lock.rb

@@ -2,6 +2,8 @@
 require 'rack/body_proxy'
 
 module Rack
+  # Rack::Lock locks every request inside a mutex, so that every request
+  # will effectively be executed synchronously.
   class Lock
     FLAG = 'rack.multithread'.freeze