pgsql: Fix erroneous documentation of synchronous_commit = remote_write

Fix erroneous documentation of synchronous_commit = remote_write.

The docs claimed that this mode only waits for the standby to receive WAL
data, but actually it waits for the data to be written out to the standby's
OS; which is a pretty significant difference because it removes the risk of
crash of the walreceiver process.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/932a271956afac90a676b946fd3e356b45581445

Modified Files
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doc/src/sgml/config.sgml            |   48 ++++++++++++++++++++--------------
doc/src/sgml/high-availability.sgml |   16 +++++++----
2 files changed, 38 insertions(+), 26 deletions(-)