PubSub: Document different PubSub received message types

[plamut]

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 the pubsub/ 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.

[plamut]

@enugentdt As the issue reporter, do you the docs changes here in this PR clear enough to answer all the questions you had when opening the issue?

[anguillanneuf]

LGTM!