Mention relevant test cases

Author: JukkaL

mypy/checker.py

@@ -4965,6 +4965,8 @@ def can_widen_in_scope(self, name: NameExpr, orig_type: Type) -> bool:
 
         Globals can only be widened from within a function if the original type
         is None (backward compat with partial type handling of `x = None`).
+
+        See test cases testNewRedefineGlobalVariableNoneInit[1-4], for example.
         """
         if (
             name.kind == GDEF