Looks like your answer is here. The author shows it available in VS2012, but gone from the list in VS2013, citing MS policy against accessing that particular folder.
[Although] this capability is gone just from the UI, you still can open appxmanifest source and manually add the capability. The result will probably be the same as before – failure of certification for individual developers, so you better stay away from this trick. Microsoft strongly recommend against using Documents Library capability, suggesting Folder and File Pickers instead.