docs: Improve A?synchronous Multimaster Replication descr.
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