#1286 Remove header comments from stubs (#1292)

- Updates documentation related to previously required comment headers.
- Removes all comment headers from stubs
- Occasionally included a header for stubs that were noted to be incomplete or contained todo's.

stdlib/2and3/sched.pyi

@@ -1,5 +1,3 @@
-# Stub for sched (Python 2 and 3)
-
 import sys
 from typing import Any, Callable, Dict, List, NamedTuple, Text, Tuple