netstat.1: Clarify -d argument

The man page states that the -d flag can be used to show the dropped
packets. But, the number of dropped input packets are always shown,
independent of the -d flag. This commit clarifies that the -d flag will
add the number of dropped output packets to the output.

MFC after: 3 days
Pull Request: #1332
Reviewed by: Alexander Ziaee

usr.bin/netstat/netstat.1

@@ -312,7 +312,7 @@ address with which they are associated.
 .It Fl b
 Show the number of bytes in and out.
 .It Fl d
-Show the number of dropped packets.
+Show the number of dropped output packets.
 .It Fl h
 Print all counters in human readable form.
 .It Fl n
@@ -381,7 +381,7 @@ Show IPv6 only.
 See
 .Sx GENERAL OPTIONS .
 .It Fl d
-Show the number of dropped packets.
+Show the number of dropped output packets.
 .It Fl M
 Use an alternative core.
 See