DOCS-15434 read write concern v0.9.0 (#49)

* DOCS-15434 read write concern faq update

* DOCS-15434 read write concern faq update

* Review feedback

Author: Dave Cuthbert

source/faq.txt

@@ -56,9 +56,21 @@ Which connection string options does ``mongosync`` allow?
 ---------------------------------------------------------
 
 ``mongosync`` requires :ref:`readConcern: "majority" <read-concern>`
-and :ref:`writeConcern: "majority" <write-concern>`. ``mongosync`` 
-overwrites any other ``readConcern`` or ``writeConcern`` and uses
-``"majority"`` instead.
+and :ref:`writeConcern: "majority" <write-concern>`. 
+
+If the ``readConcern`` is not ``majority``, ``mongosync`` returns an
+error:
+
+.. code-block:: shell
+
+   Invalid URI option, read concern must be majority
+
+If the ``writeConcern`` is not ``majority``, ``mongosync`` returns an
+error:
+
+.. code-block:: shell
+
+   Invalid URI option, write concern must be majority
 
 ``mongosync`` accepts all other :ref:`connection string options
 <mongodb-uri>`.