Documentation: Fix inconsistent section levels in class descriptions


They cause warnings when generating Qt for Python documentation.

Pick-to: 6.12
Change-Id: I4626b3e4047330b17f9449c6513983be1c9b11a3
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
commite694f33f4f1d1093eedaf1f53220450ee94fe46c
parentdf4c9817e1a8d79d293b758089ca943aef3f22c3
authorFriedemann Kleint <Friedemann.Kleint@qt.io>
committerFriedemann Kleint <Friedemann.Kleint@qt.io>
commit timeWed, 30 Sep 2026 11:07:44 +0000