Added some docs to controller runtime.

activerecord/lib/active_record/railties/controller_runtime.rb

@@ -2,7 +2,7 @@ require 'active_support/core_ext/module/attr_internal'
 
 module ActiveRecord
   module Railties
-    module ControllerRuntime
+    module ControllerRuntime #:nodoc:
       extend ActiveSupport::Concern
 
     protected
@@ -10,6 +10,9 @@ module ActiveRecord
       attr_internal :db_runtime
 
       def process_action(action, *args)
+        # We also need to reset the runtime before each action
+        # because of queries in middleware or in cases we are streaming
+        # and it won't be cleaned up by the method below.
         ActiveRecord::LogSubscriber.reset_runtime
         super
       end