Describe the enhancement or feature you'd like

In the python-dev thread, Victor expressed frustration
over lack of documentation of how to port/migrate a function
that was removed.

It would be great that going forward, when we
introduce backward incompatible changes, the documentation
on how to migrate is also included alongside
the feature implementation.

When reviewing pull requests with such changes, e.g.
when something is being deprecated or removed, reviewers
should take notice and potentially block the pull
request from getting merged, until the documentation
gets updated too.

Currently we don't have such guideline or rule. It would
be great to start recommending this practise.

We can probably add this to:

https://devguide.python.org/pullrequest/#how-to-review-a-pull-request

and/or

https://devguide.python.org/committing/#is-the-pr-ready-to-be-accepted

Thanks.