gh-144693: Clarify that PyFrame_GetBack preserves existing exception state on NULL return

Author: taegyunkim

Doc/c-api/frame.rst

@@ -50,6 +50,8 @@ See also :ref:`Reflection <reflection>`.
 
    Return a :term:`strong reference`, or ``NULL`` if *frame* has no outer
    frame.
+   Does not raise an exception, even if the return value is ``NULL``.
+   Preserves exception that was set before the call.
 
    .. versionadded:: 3.9