Describe when it may be necessary to run mergemaster before
'make buildworld'. In rare cases, mergemaster may need to be
run twice: one time before buildworld (to update /etc/group
or any other files required for a successful buildworld) and
one time after installworld (to merge all the src/etc updates
to the /etc directory of the system).

Point to /usr/src/UPDATING for details of when and why this
is necessary.

Hopefully, this will reduce the potential for confusion when
users read about mergemaster in the Handbook.

PR: docs/74231
Submitted by: Brian Candler <b.candler@pobox.com>