repo-scan: recommend running this to reinstall Git hooks after upgrading

It would be too invasive to fully automate this.

docs/upgrade.rst

@@ -232,7 +232,11 b' It is possible that an upgrade involves '
 Kallithea. As these hooks are created inside the repositories on the server
 filesystem, they are not updated automatically when upgrading Kallithea itself.
 
-To update the hooks of your Git repositories:
+To update the hooks of your Git repositories, run::
+
+    kallithea-cli repo-scan -c my.ini --install-git-hooks
+
+Or:
 
 * Go to *Admin > Settings > Remap and Rescan*
 * Select the checkbox *Install Git hooks*