Fix #celery/2269 - Better documents the queue attribute of a Task apply_async call

This is a documentation fix for https://github.com/celery/celery/issues/2269

This PR is against 3.1, but the documentation should be in both 3.1 and master I think. Celery doesn't seem to merge forward the 3.1 release branch to master and both 3.1 and master have changes that are not appropriate for the other branch.

As such, please cherry pick the commit 409ef054bd843ff418d993a678f6c9c3d3dc0ae0 onto master also.