Document how ETag caching middleware handles query parameters

doc/development/polling.md

@@ -39,6 +39,12 @@ Instead you should use polling mechanism with ETag caching in Redis.
 1. If the `If-None-Match` header does not match the current value in Redis
    we have to generate a new response, because the resource changed.
 
+Do not use query parameters (for example `?scope=all`) for endpoints where you
+want to enable ETag caching. The middleware takes into account only the request
+path and ignores query parameters. All parameters should be included in the
+request path. By doing this we avoid query parameter ordering problems and make
+route matching easier.
+
 For more information see:
 - [RFC 7232](https://tools.ietf.org/html/rfc7232)
 - [ETag proposal](https://gitlab.com/gitlab-org/gitlab-ce/issues/26926)