DOCSP-23119 consistency guarantees (#38)

* DOCSP-23119 conssitency guarantees

* DOCSP-23119 consistency guarantees

* Review feedback

Author: Dave Cuthbert

source/reference/mongosync.txt

@@ -134,6 +134,22 @@ file. For example:
    cluster0: "mongodb://192.0.2.10:27017,192.0.2.11:27017,192.0.2.12:27017"
    cluster1: "mongodb://192.0.2.20:27017,192.0.2.21:27017,192.0.2.22:27017"
 
+Consistency
+~~~~~~~~~~~
+
+``mongosync`` supports :term:`eventual consistency` on the destination
+cluster. Read Consistency is not guaranteed on the destination cluster,
+the source and destination clusters may differ at a given point in
+time. However, if new writes are halted on the source cluster, the
+source and destination clusters will eventually hold the same data. 
+
+Individual writes may be reordered or combined as they are relayed from
+source to destination. For a given document, the total number of writes
+may differ between source and destination.
+
+Transactions may appear non-atomically on the destination cluster.
+Retryable writes may not be retryable on the destination cluster. 
+
 Examples
 --------