Update pr-auto-label.yml

Signed-off-by: Matty Widdop <18513864+MattyTheHacker@users.noreply.github.com>

Author: MattyTheHacker

.github/workflows/pr-auto-label.yml

@@ -12,7 +12,14 @@ jobs:
 
     runs-on: ubuntu-latest
     steps:
-      - run: gh pr edit $PR --add-label sync
+      - name: Add "sync" label to PR
+        run: gh pr edit "$PR_URL" --add-label sync
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          PR: ${{ github.event.number }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+
+      - name: Comment on PR to explain sync label
+        run: gh pr comment "$PR_URL" --body "This pull request has been marked to **automatically sync** to its base branch. You can **disable** this behavior by removing the `sync` label."
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_URL: ${{ github.event.pull_request.html_url }}