. Use more intuitive names for describing the secondary MX and primary

MX hosts at the dialup PPP mail server part.

. Remove an explicit reference to -bd 15m and rc.conf for secondary MX

since they are too FreeBSD specific and will very often be untrue
(especially if the secondary MX is not a FreeBSD machine).

PR: docs/39757
Submitted by: Chris Pepper <pepper@rockefeller.edu>