Closes#7128.

This PR documents different PubSub received message types depending on whether a synchronous or asynchronous method is used to pull messages.

Disclaimer: I was not able to generate the docs locally due to network timeouts when downloading intersphinx inventory files from external sources. Please double check that the generated intra-doc links work.

How to test

Steps to perform:

• Generate the pubsub docs locally (from inside thepubsub/ directory):

  $ nox -f noxfile.py -s docs

• Open the generated docs for subscriber:

  $ google-chrome "file://$GOOG_CLOUD_PYTHON_DIR/pubsub/docs/_build/html//subscriber/index.html"

Expected result:
The different received message type are clearly described, the code samples are valid, and the intra-doc links work.