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