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

usr.sbin/mergemaster/mergemaster.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 29, 2015
+.Dd March 6, 2017
 .Dt MERGEMASTER 8
 .Os
 .Sh NAME
@@ -226,9 +226,10 @@ destination directory.
 .It Fl p
 Pre-buildworld mode.
 Compares only files known to be essential to the success of
-{build|install}world,
-including
-.Pa /etc/make.conf .
+{build|install}world, i.e.,
+.Pa /etc/group
+and
+.Pa /etc/master.passwd .
 .It Fl F
 If the files differ only by VCS Id ($FreeBSD)
 install the new file.