Documentation: Fix inconsistent section levels in class descriptions


They cause warnings when generating Qt for Python documentation.

Pick-to: 6.12
Change-Id: Ie3a6767be61e426de2d8462995abd4e37aa1641f
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
commit8539e605968adb15a382643609933df4cf2648f5
parenta5278e91e0a94d36b25bba1cc13b0edbc3579bd3
authorFriedemann Kleint <Friedemann.Kleint@qt.io>
committerFriedemann Kleint <Friedemann.Kleint@qt.io>
commit timeWed, 30 Sep 2026 09:55:12 +0000