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