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