D18897 added a simple port filter as sysctl tunable to siftr.
A logic bug got introduced while extracting this change from a private
branch, rendering it inoperable.
Also the intended use of the new sysctl tunable was not added
to the man page of SIFTRThe new sysctl was not added to the siftr.4 man page at the time.
This updates the man page, and removes one left over trailing whitespace.