release-process: document the rate limiting in automatic updates

We check for updates only once per day, so clarify the docs.

Change-Id: Ib669ca6ebc67bc13204996fa40e1a3a82012295e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305145
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>