Make backport responsibility clearer

[jscheffl]

As not everybody ws committer at point where we defined the policy for backporting making the message a bit more clear with a pointer to the docs.

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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[jscheffl]

I assume no backport of the backport desciption needed else we would assume we backport frmo v3.1.test to.... v2-11-test? :-D