gh-140196: Added constructor behavior changes in ast.rst for python 3.13 (GH-140243)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Doc/library/ast.rst

diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst
index 9660ad7..9b4e7ae 100644
--- a/Doc/library/ast.rst
+++ b/Doc/library/ast.rst
@@ -131,6 +131,14 @@
    Simple indices are represented by their value, extended slices are
    represented as tuples.
 
+.. versionchanged:: 3.13
+
+    AST node constructors were changed to provide sensible defaults for omitted
+    fields: optional fields now default to ``None``, list fields default to an
+    empty list, and fields of type :class:`!ast.expr_context` default to
+    :class:`Load() <ast.Load>`. Previously, omitted attributes would not exist on constructed
+    nodes (accessing them raised :exc:`AttributeError`).
+
 .. versionchanged:: 3.14
 
     The :meth:`~object.__repr__` output of :class:`~ast.AST` nodes includes