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>
diff --git a/docs/release-process.md b/docs/release-process.md
index 43209eb..3373aae 100644
--- a/docs/release-process.md
+++ b/docs/release-process.md
@@ -83,7 +83,8 @@
 *   `--repo-rev`: This tells repo which branch to use for the full project.
     It defaults to the `stable` branch (`REPO_REV` in the launcher script).
 
-Whenever `repo sync` is run, repo will check to see if an update is available.
+Whenever `repo sync` is run, repo will, once every 24 hours, see if an update
+is available.
 It fetches the latest repo-rev from the repo-url.
 Then it verifies that the latest commit in the branch has a valid signed tag
 using `git tag -v` (which uses gpg).
@@ -95,6 +96,11 @@
 
 If that tag cannot be verified, it gives up and forces the user to resolve.
 
+### Force an update
+
+The `repo selfupdate` command can be used to force an immediate update.
+It is not subject to the 24 hour limitation.
+
 ## Branch management
 
 All development happens on the `main` branch and should generally be stable.