docs: clarify path filters for changes to git submodule files

[sushant-me]

Closes #44228. Adds a note to \on.<push|pull_request|pull_request_target>.<paths|paths-ignore>\ clarifying that to detect changes to files inside a git submodule, you should filter on the path to the top-level submodule rather than paths within the submodule.

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

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[subatoi]

Thanks for submitting a PR, @sushant-me — we'll consider this in the context of the original issue, but since that wasn't approved for user contributions I'm unable to review this at this time. I'm happy to leave this open, but I'd also note that the original contributor may prefer to raise their own PR in the event that a contribution's approved. Thank you