Doc: minor language corrections

Task-number: QTBUG-35725
Change-Id: I84e47a72487b57665d0c11066cf73112a74a3529
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>