management: Document logout_all_users interaction with API keys.

Fixes part of #19397, but I'm leaving that open since we'll want to
add an option to rotate all API keys for the target users.

zerver/management/commands/logout_all_users.py

@@ -10,7 +10,12 @@ from zerver.lib.sessions import (
 
 
 class Command(ZulipBaseCommand):
-    help = "Log out all users."
+    help = """\
+Log out all users from active browser sessions.
+
+Does not disable API keys, and thus will not log users out of the
+mobile apps.
+"""
 
     def add_arguments(self, parser: ArgumentParser) -> None:
         parser.add_argument(