DOCSP-24641: add example of idFieldList for multiple fields (#142)

* DOCSP-24641: add example of idFieldList for multiple fields

Author: Chris Cho

source/configuration/write.txt

@@ -82,8 +82,15 @@ The following options for writing to MongoDB are available:
        | **Default:** ``replace``
 
    * - ``idFieldList``
-     - | Field or list of comma-separated fields to use to identify a document.
-       |
+     - | Field or list of fields by which to split the collection data. To
+         specify more than one field, separate them using a comma as shown
+         in the following example:
+
+       .. code-block:: none
+          :copyable: false
+
+          "fieldName1,fieldName2"
+
        | **Default:** ``_id``
 
    * - ``writeConcern.w``