DOCS-14182 clarify retryable writes default

Author: andf-mongodb

source/core/retryable-writes.txt

@@ -56,7 +56,16 @@ Write Acknowledgment
 Enabling Retryable Writes
 -------------------------
 
-MongoDB drivers do **not** enable retryable writes by default.
+MongoDB 3.6 introduces support for :ref:`retryable-writes`, but most
+official MongoDB 3.6-compatible drivers disable this feature
+by default. For such drivers, retryable writes can be enabled per
+connection by including the :urioption:`retryWrites=true
+<retryWrites>` option in the :ref:`connection string <mongodb-uri>`
+for that connection (see below). Refer to the
+`MongoDB Driver Documentation
+<https://docs.mongodb.com/drivers/?tck=docs_server>`_ to determine
+the correct default state of :urioption:`retryWrites` for your
+specific driver and version.
 
 MongoDB Drivers
   To enable retryable writes in MongoDB drivers, add the