Update ActionController::UrlFor request contract [ci-skip]

Using a `request` object that only "responds to the `host`,
`optional_port`, `protocol`, and `symbolized_path_parameter` methods"
has not been possible for a long time.  For example,
`symbolized_path_parameter` was renamed to `path_parameters` (without
deprecation) in 925bd97.  And
`request.routes` and `request.original_script_name` methods became
required in 87a7591.  And a
`request.engine_script_name` method became required in
4080dd2.

Therefore, this commit updates the documentation to simply require
an `ActionDispatch::Request` instance.

actionpack/lib/action_controller/metal/url_for.rb

@@ -6,10 +6,8 @@ module ActionController
   #
   # In addition to AbstractController::UrlFor, this module accesses the HTTP layer to define
   # URL options like the +host+. In order to do so, this module requires the host class
-  # to implement +env+ which needs to be Rack-compatible and +request+
-  # which is either an instance of ActionDispatch::Request or an object
-  # that responds to the +host+, +optional_port+, +protocol+, and
-  # +symbolized_path_parameter+ methods.
+  # to implement +env+ which needs to be Rack-compatible, and +request+ which
+  # returns an ActionDispatch::Request instance.
   #
   #   class RootUrl
   #     include ActionController::UrlFor