docs: specify that PyBytes_AsStringAndSize returns 0 on success (pythonGH-110888)

docs: specify return value on success

Author: lefp

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;