Add extra clarifying line in docs. [ci skip]

actionpack/lib/action_dispatch/http/request.rb

@@ -179,6 +179,7 @@ def original_url
     # The +String+ MIME type of the request
     #
     #    app.get "/articles"
+    #    app.request.media_type
     #    # => "application/x-www-form-urlencoded"
     def media_type
       content_mime_type.to_s