[3.13] gh-125436: Doc: Add missing ``allow_unnamed_section`` parameter to ``ConfigParser`` documentation (GH-125437) (#126421)

gh-125436: Doc: Add missing ``allow_unnamed_section`` parameter to ``ConfigParser`` documentation (GH-125437)

Add missing ``allow_unnamed_section`` parameter to ``ConfigParser`` doc, as well as to it's parent ``RawConfigParser``.
Split too long line on ``ConfigParser`` signature.
Add some sections about when some of ``RawConfigParser`` parameters were added.
(cherry picked from commit d9602265479bcd96dc377d92a34556baf34ac3cd)

Co-authored-by: lit <litlighilit@foxmail.com>