DOCSP-22832 bson extra elements (#229)

* added BsonExtraElements

Author: kyuan-mongodb

source/fundamentals/data-formats/pojo-customization.txt

@@ -299,6 +299,9 @@ package:
        field to BSON. You can include a discriminator to serialize POJOs
        nested within the field.
 
+   * - ``BsonExtraElements``
+     - Specifies any extra elements that aren't already mapped to a field.
+
 The following code snippet shows a sample POJO called ``Product`` that uses
 the preceding annotations.