Add comment re: Git ref to checkout step of "Manage PRs" workflow

Since it is intuitive and commonly used, the default behavior of the actions/checkout GitHub Actions action when a
workflow is triggered the "pull_request" or "push" events is well understood by any workflow maintainer. However, the
default behavior of the action when the workflow is triggered by a "pull_request_target" or "issue_comment" event (as is
the case for the "Manage PRs" workflow) is not obvious and less well known since these events are more rarely used.

It is important for the maintainer to understand which Git ref will be checked out by the actions/checkout action steps
in the "Manage PRs" workflow. Adding an explanatory comment will help to ensure they have this understanding.

Author: per1234

.github/workflows/manage-prs.yml

@@ -133,6 +133,8 @@ jobs:
       indexer-logs-urls: ${{ steps.parse-request.outputs.indexer-logs-urls }}
 
     steps:
+      # Checkout the tip of the default branch (this is the action's default ref input value when workflow is triggered
+      # by an issue_comment or pull_request_target event).
       - name: Checkout local repository
         uses: actions/checkout@v4