make [ug]id map ordering consistent with /proc/<nr>/[ug]id_map

The id ordering and case of u,g is also consistent with uidmapshift,
reducing confusion.

doc: Moved example to the the EXAMPLES section, and used values
corresponding to the defaults in the pending shadow-utils subuid patch.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>