Correct nuance of -a :service -> "*" in r314563, r314585

My attempt to correct the sender/receiver behavior was incorrect.
The source port of the sender for forwarded datagrams is filtered
with -a, and my change in r314585 didn't clarify that point at all.

Wording is based on suggestion by hrs.

MFC after: 28 days
X-MFC with: r314563, r314585
Reported by: hrs
In collaboration with: hrs, rgrimes
Sponsored by: Dell EMC Isilon