Add documentations for Document Gallery on Symbian.
Add documentations for Document Gallery on Symbian.

This patch is for:
MOBILITY-2403 Document platform support for document gallery properties.

Signed-off-by: Xizhi Zhu <6095b3dbb866d5a6b4a607e0cbd4aaa65e13c0b2@nokia.com>
Reviewed-by: Tuukka Toivonen <a598b115c645d0a951563937de34b9f14cdc01d9@nokia.com>

enthought/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,enthought/qt-mobility