Fix enum's __str__ docstring (#4827)

* fix: Enum __str__ function name

* tests: Test enum.__str__.__doc__

tests/test_enum.py

@@ -264,3 +264,8 @@ def test_docstring_signatures():
         for attr in enum_type.__dict__.values():
             # Issue #2623/PR #2637: Add argument names to enum_ methods
             assert "arg0" not in (attr.__doc__ or "")
+
+
+def test_str_signature():
+    for enum_type in [m.ScopedEnum, m.UnscopedEnum]:
+        assert enum_type.__str__.__doc__.startswith("__str__")