Document the deprecation of WFlags

Task-number: QTBUG-31737
Change-Id: I95dab7b174438b27e6fd555f88bca78099656d0c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Doc: Added a link to Qt Quick Controls Overview

Link facilitates the flow of information.

Change-Id: If84226455c35460ad226f074f9b3177eac8363e3
Reviewed-by: Martin Smith <martin.smith@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: Id43c65ddae90933db7a5a5203b4110d6939529d5