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

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

scripts/lib/clean_emoji_cache.py

@@ -4,6 +4,7 @@ import os
 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.dirname(os.path.abspath(__file__))))