doc: mention PYTEST_DISABLE_PLUGIN_AUTOLOAD in writing_plugins.rst

[bluetech]

No description provided.

[patchback]

Backport to 8.2.x: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 6d66d16 on top of patchback/backports/8.2.x/6d66d16b85b9ce87ed10ce1d6febdfdbd285abda/pr-12623

Backporting merged PR #12623 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pytest-dev/pytest.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/8.2.x/6d66d16b85b9ce87ed10ce1d6febdfdbd285abda/pr-12623 upstream/8.2.x

4. Now, cherry-pick PR doc: mention PYTEST_DISABLE_PLUGIN_AUTOLOAD in writing_plugins.rst #12623 contents into that branch:

   $ git cherry-pick -x 6d66d16b85b9ce87ed10ce1d6febdfdbd285abda

   If it'll yell at you with something like fatal: Commit 6d66d16b85b9ce87ed10ce1d6febdfdbd285abda is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 6d66d16b85b9ce87ed10ce1d6febdfdbd285abda

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR doc: mention PYTEST_DISABLE_PLUGIN_AUTOLOAD in writing_plugins.rst #12623 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/8.2.x/6d66d16b85b9ce87ed10ce1d6febdfdbd285abda/pr-12623

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[nicoddemus]

Oh bummer, haven't the time right now to do this manually -- but no big deal I guess, only did it mostly because was expecting it to be simple, regardless we will should have a new 8.3 release soon.