DOCSP-44977 notes index build behavior during sync (#463) (#467)

Author: jwilson-mdb

source/faq.txt

@@ -30,6 +30,11 @@ If you write to a synced namespace before issuing a :ref:`commit
 undefined. To ensure that you don't write to any synced namespaces, enable 
 :ref:`write blocking <c2c-write-blocking>`.
 
+.. note::
+   
+   Index builds on the destination cluster are treated as writes 
+   while ``mongosync`` is syncing.
+
 To check the value of ``canWrite``, call the :ref:`progress <c2c-api-progress>` 
 API endpoint.