docs: Add documentation for if False mypy pattern in scripts.

This should help make it clear what's going on with these scripts.

scripts/nagios/check-rabbitmq-consumers

@@ -9,6 +9,7 @@ import os
 import subprocess
 
 if False:
+    # See https://zulip.readthedocs.io/en/latest/testing/mypy.html#mypy-in-production-scripts
     from typing import Any, Dict, Optional, Union
 
 states = {