gh-125714: Doc: Explain that multiprocessing only supports unpicklable functions through the fork method

[progval]

In multiprocessing and concurrent.futures's documentations, as well as the release notes for Python 3.14.

This moves the paragraph in 3.14's release notes from the deprecations section to 'Porting to Python 3.14' because it is a breaking change, which follows a deprecation in Python 3.12.

This explicitly mentions PicklingError in this section, so it is easy to find for somehow who gets the error and searches its name through the page.

• Issue: _pickle.PicklingError on 3.14.0a1+ where it didn't before due to the start method change from fork to forkserver #125714

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📚 Documentation preview 📚: https://cpython-previews--125750.org.readthedocs.build/

[ghost]

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[progval]

Is it possible to skip the CLA for such a small change?

[rruuaanng]

You must sign the CLA, which gives us permission to distribute your work.

[gpshead]

Thanks!

[JelleZijlstra]

I am afraid you will have to sign the CLA.

[progval]

You must sign the CLA, which gives us permission to distribute your work.

I already did by submitting this PR after accepting Github's terms of service:

Whenever you add Content to a repository containing notice of a license, you license that Content under the same terms, and you agree that you have the right to license that Content under those terms.

Nonetheless, I'm fine with the CLA on principle, but I'm not willing to give out the personal info the cla-bot or the form are asking for.

[willingc]

I'm going to go ahead and close this PR if there is no signed CLA. If someone wishes to work with @progval to take over this work, we can reopen the PR. Thanks @progval for your effort.