update wording and labels of repo-sync PRs (#15783)

Author: zeke

.github/workflows/repo-sync.yml

@@ -41,8 +41,8 @@ jobs:
         source_branch: repo-sync
         destination_branch: main
         pr_title: "repo sync"
-        pr_body: "This is an automated pull request to sync changes between the public and private docs repos.\n\n:robot: Please let a bot do the merging!"
-        pr_label: "automerge"
+        pr_body: "This is an automated pull request to sync changes between the public and private repos.\n\n:robot: This pull request should be merged (not squashed) to preserve continuity across repos, so please let a bot do the merging!"
+        pr_label: automerge,autoupdate
         github_token: ${{ secrets.OCTOMERGER_PAT_WITH_REPO_AND_WORKFLOW_SCOPE }}
 
     - name: Find pull request