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

usr.bin/netstat/netstat.1

@@ -28,7 +28,7 @@
 .\"	@(#)netstat.1	8.8 (Berkeley) 4/18/94
 .\" $FreeBSD$
 .\"
-.Dd May 27, 2022
+.Dd February 18, 2023
 .Dt NETSTAT 1
 .Os
 .Sh NAME
@@ -284,6 +284,14 @@ The network addresses of the interface
 and the maximum transmission unit
 .Pq Dq mtu
 are also displayed.
+If both
+.Fl i
+and
+.Fl I
+are specified,
+.Fl I
+overrides any instances of
+.Fl i .
 .Bl -tag -width indent
 .It Fl 4
 Show IPv4 only.
@@ -432,6 +440,14 @@ See
 .Ek
 .Xc
 Display per-interface statistics for each network protocol.
+If both
+.Fl i
+and
+.Fl I
+are specified,
+.Fl I
+overrides any instances of
+.Fl i .
 .Bl -tag -width indent
 .It Fl 4
 Show IPv4 only