Merge pull request #20314 from vngrs/add_doc_about_form_data_method

Documentation for ActionDispatch::Request form_data? method [ci skip]

actionpack/lib/action_dispatch/http/request.rb

@@ -295,6 +295,8 @@ def body
       end
     end
 
+    # returns true if request content mime type is
+    # +application/x-www-form-urlencoded+ or +multipart/form-data+.
     def form_data?
       FORM_DATA_MEDIA_TYPES.include?(content_mime_type.to_s)
     end