Обсуждение: pgsql: doc: improve description of synchronous_commit modes

doc:  improve description of synchronous_commit modes

Previously it wasn't clear exactly what each of the synchronous_commit
modes accomplished.  This clarifies that, and adds a table describing it.
Only backpatched through 9.6 since 9.5 doesn't have all the options.

Reported-by: kghost0@gmail.com

Discussion: https://postgr.es/m/159741195522.14321.13812604195366728976@wrigleys.postgresql.org

Backpatch-through: 9.6

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ed4a1c873649b4ababbe5b92ad17e9a29215f168

Modified Files
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doc/src/sgml/config.sgml  | 144 ++++++++++++++++++++++++++++++++++++----------
src/include/access/xact.h |   3 +-
2 files changed, 116 insertions(+), 31 deletions(-)