PubSub: Document FlowControl settings

[plamut]

Closes #7043.

This PR makes sure that FlowControl settings are properly documented in the generated documentation.

How to test

Steps to perform:

(from inside the pubsub/ directory)

• Install the synthtool and run it,
• Generate the pubsub docs locally:

  $ nox -f noxfile.py -s docs
  

• Open the generated docs for FlowControl:

  $ chromium-browser "file://$GOOG_CLOUD_PYTHON_DIR/pubsub/docs/_build/html/types.html#google.cloud.pubsub_v1.types.FlowControl"
  

Expected result:
FlowControl settings have nice descriptions (as opposed to the current docs).

[anguillanneuf]

The generated docs look good to me.

Nit: I see the word "Property" in front of each property in the generated docs. Not a big deal, but would be great if we can make it consistent with the rest of the documentation.

[plamut]

The "property" prefix seems to be added to all namedtuple members, including those that do not have their docstrings modified, e.g. types.BatchSettings. It's probably the Sphinx itself that adds them, and the output might depend on the latter's version.

How are the current online docs generated and published anyway? Would a result be different if that was used instead of the local docs build?

FWIW, I get the same result (the "property" prefix) if building the docs locally on the master branch, making me believe that it's likely something unrelated to the changes in this PR ...

[anguillanneuf]

LGTM!