Documentation: Fix inconsistent section levels in class descriptions


They cause warnings when generating Qt for Python documentation.

Pick-to: 6.12
Change-Id: I204c8d00f282f0c30186caaecf791d4513b01c81
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
commit75142dd8ceed17ec8bcf6181aa360614a5c96efe
parenta8fdbc852bed434a11f3b1adb6668bb62914499e
authorFriedemann Kleint <Friedemann.Kleint@qt.io>
committerFriedemann Kleint <Friedemann.Kleint@qt.io>
commit timeWed, 30 Sep 2026 09:55:31 +0000