DOCSP-44180: Clarify _id field behavior for nested documents (mongodb#323)

(cherry picked from commit 73e4e14)

Author: mcmorisi

source/fundamentals/serialization/poco.txt

@@ -243,6 +243,11 @@ The following code sample maps the ``Identifier`` property to the
    (for example, if your POCO includes properties named ``Id`` and ``_id``), 
    the driver throws a ``BsonSerializationException``.
 
+.. note:: Nested Document Ids
+
+   The ``_id`` field mapping logic described in this section only applies to the
+   root document and does not apply to nested documents.
+
 Omit Empty Fields
 ~~~~~~~~~~~~~~~~~