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: I2ec9242c0bbb4e66d6c27be6dba8ab0b365bb17f
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>