The docs for sync and async multimaster replication were unclear about
when to use it, and when it has benefits;  this change clarifies that.

Reported-by: juha-pekka.eloranta@reaktor.fi

Discussion: https://postgr.es/m/156856543824.1274.12180817186798859836@wrigleys.postgresql.org

Backpatch-through: 9.4