Clarify unpacked type handling in expandtype.py

Add comment to clarify behavior during semantic analysis.

Author: hyperkai

mypy/expandtype.py

@@ -541,6 +541,7 @@ def visit_tuple_type(self, t: TupleType) -> Type:
             if isinstance(item, UnpackType):
                 unpacked = get_proper_type(item.type)
                 if isinstance(unpacked, Instance):
+                    # expand_type() may be called during semantic analysis, before invalid unpacks are fixed.
                     if unpacked.type.fullname != "builtins.tuple":
                         return t.partial_fallback.accept(self)
                     if t.partial_fallback.type.fullname != "builtins.tuple":