Update per user topic registration request description

Sync migrated to standalone invalidations using FCM registration token
instead of per-user topics. This CL updates the description and removes
Sync from it.

Fixed: 40252065
Change-Id: I8c5f5ecc8e4c5e70b300f4d6ce848805c66d8dbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5398178
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Theodore Olsauskas-Warren <sauski@google.com>
Cr-Commit-Position: refs/heads/main@{#1279553}