`on.<push|pull_request|pull_request_target>.<paths|paths-ignore>` should indicate how to match on git submodules

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#onpushpull_requestpull_request_targetpathspaths-ignore

What part(s) of the article would you like to see updated?

Add a note that for changes to files in git submodules, the path to the changed top level submodule should be used (instead of the path to files within the submodule).

Additional information

It took a couple of attempts for a dev to figure out the correct way to make this work: https://github.com/PowerDNS/pdns/compare/56c6b1011d488e26673320b9d54aa4a3bce6a28d..a13665dcb8f9d57033820f53d83aa3e387ca9ecb