Improve boolean indexing docs
- [ X] closes #42384
- [ X] tests added / passed
- [ X] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ X] whatsnew entry

This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but it appears to have gone stale. This was close, but if you could push some small updates based on the review we would be happy to accept this PR. Closing in the meantime.