mergemaster: fix description of -p

-p only handles updating /etc/master.passwd and /etc/group . No more,
no less.

Also, mergemaster (and no other portions of the vanilla FreeBSD build
process) should be messing with __MAKECONF or SRCCONF as part of the
installworld or distribution process. Don't insinuate that mergemaster
does that as it's a false claim.

MFC after: 1 week
Sponsored by: Dell EMC Isilon