PubSub: Document batch settings, make synth operations idempotent

[plamut]

This PR makes sure that BatchSettings settings are properly documented in the generated documentation. It also fixes multiple repeated blocks for adding FlowControl docstrings (supersedes #8447).

How to test

Steps to perform:

(from inside the pubsub/ directory)

• Install the synthtool and run it.
• Run the synthtool a few more times.
• Generate the pubsub docs locally:

  $ nox -f noxfile.py -s docs
  

• Open the generated docs for BatchSettings:

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

Actual result (before the fix):

• A new code block for injecting FlowControl docstrings is added after every synthtool run, growing the file indefinitely.
• BatchSettings attributes only have uninformative generic descriptions.

Expected result (after the fix):

• No new code is added to types.py by the synthtool.
• The BatchSettings type and its attributes have nice descriptions (as opposed to the current docs).