It's an interesting read. Recently I read another article on CNN about how the Marines at Arlington National Cemetery come out every night and make photo records of all the new burials that went on that day. They use....you guessed it....iPhones. On average about 45 burials occur every day and therefore documenting these is critical for maintaining the whereabouts of those laid to rest there.
The Old Guard at Arlington National Cemetery are the 3d US Infantry from Ft. Meyer which come from the US Army, not the Marine Corps.
A lot depends upon what 'documentation' means.
Documentation used to be done by the Signal Corps assigned to Ft. Meyer, too, with Nikons or Hasselblads. I doubt it has changed, but they could use what ever they want.
If the article referred to something else, such as grave placement and registration, that would be something else, but that also falls under what used to be called Mortuary Affairs. I don't know what it might be called now.