Document CheckBox.
Document CheckBox.

Change-Id: Iaa60ffde5ff4c0d3a6d8a9c1277fa77ccbbb0c24
Reviewed-by: Venugopal Shivashankar <5756f69ec86de98024d1d912258c6908b6f2ee8f@digia.com>
Reviewed-by: Jerome Pasion <c10995d24f6b454503389e8d00d837aef038985d@digia.com>

kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,jfaust/qtquickcontrols,kobolabs/qtquickcontrols,jfaust/qtquickcontrols,kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,jfaust/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,richardmg/qtcomponents,drgvond/qtquickcontrols