netstat.1: Clarify -i and -I priorities

netstat(1) allows to specify both -i (all interfaces) and -I <ifname>.
However, when both are specified, -I always overrides -i.
Add a comment where appropriate the same way we do in rm(1) for -f and -i.

PR: 202708
Reported by: demon@
Approved by: manpages (pauamma@)
Differential Revision: https://reviews.freebsd.org/D38654