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
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b88b929a707145964b707257049f8dbfd58f0da5

Modified Files
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doc/src/sgml/backup.sgml              |  3 ++-
doc/src/sgml/ref/pg_basebackup.sgml   | 10 +++++++++-
src/bin/pg_basebackup/pg_basebackup.c | 11 +++++++++++
3 files changed, 22 insertions(+), 2 deletions(-)