fixes #502: include infromation about auto-closure of bpo isssues

[kumiDa]

Fixes #502

Feature that was included in PEP-595 to automatically close issue
in the bug tracker when a PR with commit message including words like-
close, closes, closed, closing, fix, fixes or fixed being used with
issue number is merged is documented in the devguide.

[kumiDa]

@hugovk, the CLA is signed...still the CLA signing status is stated as expected.
Can you please help me with this.

[JelleZijlstra]

This still talks about bpo, which is no longer relevant.

Additionally, it would fit better at https://devguide.python.org/gitbootcamp/#accepting-and-merging-a-pr where we already talk about commit messages.

[ezio-melotti]

Closing since we now migrated to GitHub.

[gvanrossum]

But GitHub has a similar feature. Is that documented?

[ezio-melotti]

Is that documented?

Apparently not.

Recently there was a discussion about the fact that using the auto-closing keywords is not always desirable, e.g.:

• when the PR needs to be backported
• when multiple PRs address the same issue

In these cases it might be better to avoid keywords and just close the issue manually once all the relevant PRs are merged. Both the keywords and recommendations on where they should be used/avoid can be documented.

I would really like to see sections with basic instructions/tips/reference for all the tools we use, but I'm still not sure what would be the best way to organize this.