Back-patch checkpoint clarification docs and pg_basebackup updates

This backpatches 51e26c9c and 7220c7b3, including both documentation
updates clarifying the checkpoints at the beginning of base backups and
the messages in verbose and progress mdoe of pg_basebackup.

Author: Michael Banck
Discussion: https://postgr.es/m/21444.1488142764%40sss.pgh.pa.us