DOCS-14523 update _id fields

Author: ajhuh-mdb

source/core/document.txt

@@ -220,7 +220,7 @@ The ``_id`` field has the following behavior and constraints:
   first, then the server will move the field to the beginning.
 
 - The ``_id`` field may contain values of any :doc:`BSON data type
-  </reference/bson-types>`, other than an array.
+  </reference/bson-types>`, other than an array, regex, or undefined.
 
   .. warning:: To ensure functioning replication, do not store values
      that are of the BSON regular expression type in the ``_id``