ifconfig.8: Document "ifconfig -g groupname"

"ifconfig -g groupname" prints a list of interface names,
which could be confusing, because it differs from
the behavior of "ifconfig -a -g groupname".

While here, add two examples showing the difference between
"ifconfig -a -g groupname" and "ifconfig -g groupname".

Fixes: 0dad3f0e1512 Import interface groups from OpenBSD.
MFC after: 2 weeks