CBL REST API: _replicate doc missing most of the JSON options

First comment:

The REST API doc for [_replicate](http://developer.couchbase.com/mobile/develop/references/couchbase-lite/rest-api/server/post-replicate/index.html) only covers three of the many properties available in the JSON body.

Most importantly, it doesn't document how to cancel a replication.

CouchDB documentation is [here](http://docs.couchdb.org/en/latest/api/server/common.html#replicate), but we don't support all those options and we add some of our own.

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/137