docs(topology): some more module docstrings

[rwbarton]

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list

[PatrickMassot]

Thanks Reid! Could you please try to include [ci skip] in your commit messages when touching only doc? This speeds up the PR queue for everyone.