Documentation: Fix inconsistent section levels in class descriptions


They cause warnings when generating Qt for Python documentation.

Pick-to: 6.12
Change-Id: If31563d14454152ad649e6eae14bcc736cf9cde7
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
commitc295a62d99163d4b70d5530757c4f342e3024dbf
parenta8fb165ca8ec918812f8a3c49e1144b0a8a975b5
authorFriedemann Kleint <Friedemann.Kleint@qt.io>
committerFriedemann Kleint <Friedemann.Kleint@qt.io>
commit timeWed, 30 Sep 2026 09:55:26 +0000