chore(ci): add note to PR body created by update_librarian_googleapis.yaml (#13436)

Add a note to the PR body created by update_librarian_googleapis.yaml,
with instruction to either close or merge in PR.

For simplicity, this workflow does not update the PR once created. This
instruction let people know reading the PR what to do to unblock the
next scheduled run if they don't want to merge in.

Author: zhumin8

.github/workflows/update_librarian_googleapis.yaml

@@ -128,7 +128,10 @@ jobs:
       env:
         GH_TOKEN: ${{ secrets.CLOUD_JAVA_BOT_GITHUB_TOKEN }}
         PR_TITLE: "chore: update googleapis commitish to ${{ steps.commit.outputs.short_commit }}"
-        PR_BODY: "Updated googleapis commitish in librarian.yaml and generation_config.yaml to https://github.com/googleapis/googleapis/commit/${{ steps.commit.outputs.new_commit }}"
+        PR_BODY: |
+          Updated googleapis commitish in librarian.yaml and generation_config.yaml to https://github.com/googleapis/googleapis/commit/${{ steps.commit.outputs.new_commit }}
+
+          💡 **Note:** Please merge or close this PR so that the daily update workflow can continue to run successfully the next day. Alternatively, you can manually trigger the workflow once this PR is merged or closed.
       run: |
         set -x