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 925bd975.  And
`request.routes` and `request.original_script_name` methods became
required in 87a75910.  And a
`request.engine_script_name` method became required in
4080dd2f.

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

(cherry picked from commit d69501a3)