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: I2d91518900c9b60ee8e8a8f549c88a1d50632b3d
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>