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: I2e946fda0bd9a2117f8e9b2fb300df9bf0a98a6c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>