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
Reviewed by: imp, Alexander Ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1332