docs: Add a warning about editing mypy stubs and the daemon.

docs/testing/mypy.md

@@ -95,6 +95,10 @@ everything in the third-party module as an `Any`, which is the right
 model (one certainly wouldn't want to need stubs for everything just
 to use `mypy`!), but means the code can't be fully type-checked.
 
+**Note**: When editing stubs, we recommend using
+`tools/run-mypy --no-daemon`, because the mypy daemon's caching
+system has some bugs around editing stubs that can be confusing.
+
 ## `type_debug.py`
 
 `zerver/lib/type_debug.py` has a useful decorator `print_types`.  It

mypy.ini

@@ -2,6 +2,8 @@
 # Logistics of what code to check and how to handle the data.
 scripts_are_modules = True
 show_traceback = True
+# See https://zulip.readthedocs.io/en/latest/testing/mypy.html#mypy-stubs-for-third-party-modules
+# for notes on how we manage mypy stubs.
 mypy_path = stubs/
 cache_dir = var/mypy-cache