Add note about git submodule path matching for workflow triggers

[trustdemons05]

Closes #44234

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Source	Review	Production	What Changed
actions/how-tos/write-workflows/choose-when-workflows-run/trigger-a-workflow.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	from reusable
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	from reusable

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