add details on root flag related to no_root_squash

doc/administration/high_availability/nfs.md

@@ -25,7 +25,9 @@ options:
   errors when the Omnibus package tries to alter permissions. Note that GitLab
   and other bundled components do **not** run as `root` but as non-privileged
   users. The recommendation for `no_root_squash` is to allow the Omnibus package
-  to set ownership and permissions on files, as needed.
+  to set ownership and permissions on files, as needed. In some cases where the
+  `no_root_squash` option is not available, the `root` flag can achieve the same
+  result.
 - `sync` - Force synchronous behavior. Default is asynchronous and under certain
   circumstances it could lead to data loss if a failure occurs before data has
   synced.