nvmecontrol.8: Explain non-operational power modes

nvmecontrol power -l ... lists the available power modes.
Non-operational modes are marked with an asterisk. While here,
add <device-id | namespace-id> to the "nvmecontrol power" synopsis.

MFC after: 3 days
Reviewed by: dab, imp, michaelo, ziaee
Differential Revision: https://reviews.freebsd.org/D58480

(cherry picked from commit 868158f7fd2a172ef22f1b6b682cc1d38e54cf63)