[doc] Clarify MRO precedence in descriptor super binding section (GH-29539) (GH-29669)

A similar sentence is present in the 'Invocation from super' section of
the descriptor HOWTO, where it is already correct.
(cherry picked from commit ee49484c0f0d0d79e8fc40835da10b78f89ae503)

Co-authored-by: Jouke Witteveen <j.witteveen@gmail.com>