libgeom(3): apply minor polish

• Use .Dv when mentioning NULL per mdoc(7).
• Reword g_device_path, g_open_by_ident, and g_providername's descriptions so they're less wordy.
• Fix a typo in g_device_path (can not -> cannot).

MFC after: 5 weeks
Tested with: igor, make manlint
Sponsored by: Dell EMC Isilon