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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ee59f669b666e62b7e769ece97ef74ef621acd8d

Modified Files
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doc/src/sgml/config.sgml  | 144 ++++++++++++++++++++++++++++++++++++----------
src/include/access/xact.h |   4 +-
2 files changed, 116 insertions(+), 32 deletions(-)