Add note to docs about `Gitlab::SafeRequestStore`

doc/development/merge_request_performance_guidelines.md

@@ -168,6 +168,7 @@ user objects for every username we can remove the need for running the same
 query for every mention of `@alice`.
 
 Caching data per transaction can be done using
-[RequestStore](https://github.com/steveklabnik/request_store). Caching data in
-Redis can be done using [Rails' caching
+[RequestStore](https://github.com/steveklabnik/request_store) (use
+`Gitlab::SafeRequestStore` to avoid having to remember to check
+`RequestStore.active?`). Caching data in Redis can be done using [Rails' caching
 system](http://guides.rubyonrails.org/caching_with_rails.html).