Add versionchanged tag

Author: hynek

src/attr/validators.py

@@ -79,6 +79,7 @@ def disabled():
         This context manager is not thread-safe!
 
     .. versionadded:: 21.3.0
+    .. versionchanged:: 26.1.0 The contextmanager is nestable.
     """
     prev = get_run_validators()
     set_run_validators(False)