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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actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.223.213.203.193.183.17	fpt ghec ghes@ 3.223.213.203.193.183.17

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