doc: make createPushResponse() more detailled

We don't know what will return when successful or failure for
the callback of the function. So this commit makes it more detailled.

PR-URL: https://github.com/nodejs/node/pull/22366
Refs: https://github.com/nodejs/node/issues/22322


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>