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/node_cache.py

@@ -4,6 +4,7 @@ import hashlib
 import json
 
 if False:
+    # See https://zulip.readthedocs.io/en/latest/testing/mypy.html#mypy-in-production-scripts
     from typing import Optional, List, IO, Tuple, Any
 
 from scripts.lib.zulip_tools import subprocess_text_output, run