pythongh-128481: indicate that the default value for FrameSummary.end_lineno changed in 3.13 (python#130755)

The value taken by `FrameSummary.end_lineno` when passing `end_lineno=None` changed in pythongh-112097.

Previously, a `end_lineno` could be specified to be `None` directly but since 939fc6d, passing None makes
the constructor use the value of `lineno` instead.

Author: picnixz

Doc/library/traceback.rst

@@ -551,6 +551,9 @@ in a :ref:`traceback <traceback-objects>`.
       The last line number of the source code for this frame.
       By default, it is set to ``lineno`` and indexation starts from 1.
 
+      .. versionchanged:: 3.13
+         The default value changed from ``None`` to ``lineno``.
+
    .. attribute:: FrameSummary.colno
 
       The column number of the source code for this frame.