Doc: check language and compliance with Writing Guidelines

Task-number: QTBUG-30318

Change-Id: I6117c41344f1a5015a1fa2d4a59950306a719313
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>