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

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

scripts/purge-old-deployments

@@ -5,6 +5,7 @@ import subprocess
 import sys
 
 if False:
+    # See https://zulip.readthedocs.io/en/latest/testing/mypy.html#mypy-in-production-scripts
     from typing import Set
 
 ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))