docs: clarify path filters for git submodule changes

Path filters in `on.<push|pull_request|pull_request_target>.<paths|paths-ignore>`
are compared against the parent repository's diff, which records a submodule as a
single entry at the submodule's own path. Filtering on a path inside the submodule
therefore never matches, because the parent repository does not track those paths.

Author: sushant-me

content/actions/reference/workflows-and-actions/workflow-syntax.md

@@ -175,6 +175,8 @@ on:
 ### Git diff comparisons
 
 {% data reusables.actions.workflows.triggering-a-workflow-paths5 %}
+> [!NOTE]
+> **Changes to files in a git submodule:** For {% data variables.product.prodname_actions %} to detect changes to files inside a git submodule, filter on the path to the top-level submodule itself, not on paths within the submodule. For example, if a submodule is checked out at `submodules/my-library`, use `paths: - submodules/my-library/**`. The paths to individual files inside the submodule are not tracked by the parent repository's workflow filters.
 
 ## `on.schedule`