DOCSP-45737: Clarify that users must specify id for bulk inserted documents

Author: mcmorisi

source/write/bulk-write.txt

@@ -67,6 +67,11 @@ The following example creates an instance of ``InsertOne``:
 
 To insert multiple documents, create an instance of ``InsertOne`` for each document.
 
+.. note::
+
+   You must specify the ``_id`` field for each document you insert. If you don't, the driver
+   throws a duplicate key error.
+
 Update Operations
 ~~~~~~~~~~~~~~~~~