Fix some typos in documentation.

Fix some spelling and grammatical errors in comments that show up in Qt documentation. No changes to code.

Change-Id: Ieb7be17bd1d50751f00620f7ac28d09355856ac0
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>