pgsql: Back-patch checkpoint clarification docs and pg_basebackupupdat

Back-patch checkpoint clarification docs and pg_basebackup updates

This backpatches 51e26c9 and 7220c7b, 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

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/bd34e7f19b404d025d5b36bac42bcfecf2c09bc5

Modified Files
--------------
doc/src/sgml/ref/pg_basebackup.sgml   | 10 +++++++++-
src/bin/pg_basebackup/pg_basebackup.c | 11 +++++++++++
2 files changed, 20 insertions(+), 1 deletion(-)