netstat.1: Clarify -i and -I priorities

[fernape]

Summary

Comes from PR 202708

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

Current behavior from current:

$ netstat -i
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs
Coll
re0    1500 <Link#1>      dc:fe:07:4b:d6:14        0     0     0        0     0 0
lo0   16384 <Link#2>      lo0                      0     0     0        0     0 0
lo0       - localhost     localhost                0     -     -        0     - -
lo0       - fe80::%lo0/64 fe80::1%lo0              0     -     -        0     - -
lo0       - your-net      localhost                0     -     -        0     - -
wlan0  1500 <Link#3>      10:02:b5:5c:f3:d7    17776     0     0    13625     2 0
wlan0     - 192.168.1.0/2 192.168.1.40         16027     -     -    13558     - -

$ netstat -I wlan0
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs Coll
wlan0  1500 <Link#3>      10:02:b5:5c:f3:d7    17790     0     0    13637     2 0
wlan0     - 192.168.1.0/2 192.168.1.40         16040     -     -    13569     - -

$ netstat -I wlan0 -i
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs Coll
wlan0  1500 <Link#3>      10:02:b5:5c:f3:d7    17804     0     0    13642     2 0
wlan0     - 192.168.1.0/2 192.168.1.40         16045     -     -    13574     - -

$ netstat -i -I wlan0
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs Coll
wlan0  1500 <Link#3>      10:02:b5:5c:f3:d7    18051     0     0    13660     2 0
wlan0     - 192.168.1.0/2 192.168.1.40         16066     -     -    13591     - -

Test Plan

Apply patch.

• man netstat.1 renders the page properly
• igor and mandoc -T lint clean.

[fernape]

Thanks for the review!