Doc: Fix some documentation warnings

- Can't link to 'QSlider'
- Can't link to 'QImage'

Change-Id: Ifa1a27ca5224959d6dbb4c0055a56114740469f0
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>