Fix RestClient::Request docs when used with yard

This moves a backwards compatibility comment.  It's currently replacing
the class documentation, which means that most of the docs for this
class are missing on rubydoc.info -
http://rubydoc.info/gems/rest-client/RestClient/Request
Merge pull request #319 from alext/fix_restclient_response_docs

Fix RestClient::Request docs when used with yard
Rename Platform.mac? to .mac_mri? for clarity.

This better reflects the fact that the method is expected to return
false on non-MRI engines even on OS X.