Docs: Improve replication backoff discussion

This page: http://www.couchbase.com/docs/couchbase-manual-2.0/couchbase-admin-tasks-replica-backoff.html

-We really don't want to refer to "replica nodes" and "source nodes" because it creates a lot of confusion, especially since other technologies use a replica/slave topology. We have replica vbuckets that are spread across the cluster along with the active vbuckets.
-"By default replica nodes will send backoff messages to source nodes when the disk write queue on the replica node contains one million items"...it's actually 1M or 10%, whichever is higher...that's not just configurable, it's how it works.
-"Both the reads/writes as well as replicated data must wait in disk write queue to be written to disk"...reads don't wait in any queue, and reads have nothing to do with replication
-It would be very helpful to describe and/or link to a section on how to monitor the replication backoff when it is happening