Improve CouchDB Replication Protocol definition

This updated version contains detailed information about how CouchDB
Replicator uses public HTTP API to synchronize documents between two
databases.

While this document doesn't describes all details about Replicator
workflow, the provided information is quite enough to easy implement
minimal required HTTP API for custom services to be able act as peer
in pull and push replications which runs by couch_replicator app.

COUCHDB-1824

Thanks to Andreas Wenk for help with typos!