Add a comment stating that the proc is evaluated in the controller context

Author: Sergei Maximov

lib/jsonapi/rails/renderer.rb

@@ -28,6 +28,8 @@ def self.render(errors, options)
     # @api private
     def rails_renderer(renderer)
       proc do |json, options|
+        # Renderer proc is evaluated in the controller context, so it
+        # has access to the request object.
         reverse_mapping = request.env[ActionController::REVERSE_MAPPING_KEY]
         options = options.merge(_reverse_mapping: reverse_mapping)
         json = renderer.render(json, options) unless json.is_a?(String)