DOCS 16510 default write concern info (#5525) (#5546)

* DOCS-16510 default write concern

* DOCS-16510 default write concern

Author: ltran-mdb2

source/reference/write-concern.txt

@@ -69,8 +69,15 @@ Write concern can include the following fields:
 ~~~~~~~~~~~~
 
 The ``w`` option requests acknowledgment that the write operation has
-propagated to a specified number of :binary:`~bin.mongod` instances or to
-:binary:`~bin.mongod` instances with specified tags.
+propagated to a specified number of :binary:`~bin.mongod` instances or
+to :binary:`~bin.mongod` instances with specified tags. If the write
+concern is missing the ``w`` field, MongoDB sets the ``w`` option to the
+default write concern. 
+
+.. note::
+
+  If you use the :dbcommand:`setDefaultRWConcern` to set the default
+  write concern, you must specify a ``w`` field value. 
 
 Using the ``w`` option, the following ``w: <value>`` write concerns are
 available: