[git_cl] Improve docstring

Bug: b/351071334
Change-Id: Id28e9d87c517ea0ad6c4a939ce85b52fc6f55810
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5723090
Commit-Queue: Allen Li <ayatane@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -3772,7 +3772,8 @@ class GitAuthConfigChanger(object):
     def apply_global(self, cwd: str) -> None:
         """Apply config changes to the global (user) Git config.
 
-        This uses the instance's mode as a default for the Gerrit host.
+        This will make the instance's mode (e.g., SSO or not) the global
+        default for the Gerrit host, if not overridden by a specific Git repo.
         """
         self._apply_global_cred_helper(cwd)
         self._apply_global_sso(cwd)