CTest's DartMeasurement test output scanning should be documented

As recently raised on stackoverflow, the way that ctest scans the test output looking for DartMeasurement and DartMeasurementFile tags is not documented. It has been used for quite some time by projects like Vtk by the look of it and CDash presents images especially well when added to the test output via this method. This behavior should be documented so that it can be more widely used.