MFC r314830:

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 r288381:

All supported releases have the -m support from r186678, so remove the
mention of it and reword this a bit to remove 'you'.

MFC r288381:

All supported releases have the -m support from r186678, so remove the
mention of it and reword this a bit to remove 'you'.