Document how documentation changes are backported

[JohnVeness]

Your Godot version: 3.4

Issue description: In the "Contributing to the documentation" doc (URL below) it says:

Pull Requests should use the master branch by default. Only make Pull Requests against other branches (e.g. 2.1 or 3.0) if your changes only apply to that specific version of Godot.

What I'm unsure about is the process of how master changes are backported to older branches. Is this something that we (as contributors) should do, or at least suggest? Looking in various PRs I see cherrypick labels, but it's unclear to me who decides and implements these.

It would be great to update the below URL with more information on this process. Thanks :)

URL to the documentation page (if already existing): https://docs.godotengine.org/en/latest/community/contributing/contributing_to_the_documentation.html

[Calinou]

What I'm unsure about is the process of how master changes are backported to older branches. Is this something that we (as contributors) should do, or at least suggest?

The rule is that PRs that are fully applicable to an older branch can be cherry-picked to the stable branches. If a PR needs to have changes before it's applicable to an older branch, then you should open another pull request targeting one of the versioned stable branches (3.5 or 3.4, not stable as stable is mirroring 3.4 currently).

When opening a second PR against a stable branch, please link to the original master PR for convenience like this: https://github.com/godotengine/godot-docs/pull/5963

If a PR is not applicable to stable branches at all (or master if it targets 3.4/3.5), then you don't need to open a second PR and no cherrypick label is added.

Looking in various PRs I see cherrypick labels, but it's unclear to me who decides and implements these.

Core contributors handle cherry-picking. If you see a cherrypick label, you don't have to do anything specific.