DOCSP-45058-mongosh-undefined-behavior (#364)

* DOCSP-45058-mongosh-undefined-behavior

* render fixes

* render fix

* make docs plural

* reword

* clarify

* reviewer changes

* .

Author: amalhotra-mdb

source/reference/compatibility.txt

@@ -202,6 +202,30 @@ in ``mongosh`` better align with the types used by the MongoDB Drivers.
    For more information on managing types, refer to the
    :manual:`schema validation overview </core/schema-validation>`.
 
+Undefined Values
+----------------
+
+The undefined BSON type is `deprecated <https://bsonspec.org/spec.html>`__ in
+:binary:`bin.mongosh`. If you insert a document with the undefined JS type in
+:binary:`bin.mongosh`, your deployment replaces the undefined value in your 
+document with the BSON null value. There is no supported way of inserting undefined
+values into your database using :binary:`bin.mongosh`.
+
+For example, consider running the following code to insert a document in
+:binary:`bin.mongosh`:
+
+.. code-block:: javascript
+
+   db.people.insertOne( { name : "Sally", age : undefined } )
+
+When you run this code in :binary:`bin.mongosh`, the shell inserts 
+the following document: 
+
+.. code-block:: javascript
+   "copyable: false
+
+   { name : "Sally", age : null }
+
 Object Quoting Behavior
 -----------------------