Doc: Adding import statement for in Qt Multimedia QML Types

Change-Id: I17e108c5d8524fe428ab8f0ca14130cd4bb78d2d
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>