nvmecontrol: Flesh out nvmecontrol format information

The format command takes a number of different parameters. Include a
brief summary of what the values mean, though since the driver's support
for metadata is at best weak, 0's are almost always used for values
other than -f format. Add an example that ties it all together.

Sponsored by: Netflix
Reviewed by: pauamma@gundo.com, chuck
Differential Revision: https://reviews.freebsd.org/D44958