[3.12] docs: specify that PyBytes_AsStringAndSize returns 0 on success (GH-110888) (GH-111690)

(cherry picked from commit 9f33ede)

Co-authored-by: lefp <70862148+lefp@users.noreply.github.com>

Author: miss-islington

Doc/c-api/bytes.rst

@@ -155,6 +155,7 @@ called with a non-bytes parameter.
 
    Return the null-terminated contents of the object *obj*
    through the output variables *buffer* and *length*.
+   Returns ``0`` on success.
 
    If *length* is ``NULL``, the bytes object
    may not contain embedded null bytes;