Update whats-new.txt

Author: johnnyshields

source/whats-new.txt

@@ -148,6 +148,23 @@ Removal of Deprecated Class ``Mongoid::Errors::InvalidStorageParent``
 The deprecated class ``{+odm+}::Errors::InvalidStorageParent`` has been
 removed.
 
+``around_*`` Callbacks for Embedded Documents are Ignored by Default
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+{+odm+} v8.x and older allow user to define ``around_*`` callbacks for embedded
+documents. Starting in v9.0, by default, these callbacks are ignored and will
+not be executed. A warning will be printed to the console if such callbacks are defined.
+
+If you want to restore the old behavior, you can set
+``Mongoid.around_callbacks_for_embeds`` to true in your application.
+
+.. note::
+
+   Enabling ``around_*`` callbacks for embedded documents is not recommended
+   as it may cause ``SystemStackError`` exceptions when a document has many
+   embedded documents. See `MONGOID-5658
+   <https://jira.mongodb.org/browse/MONGOID-5658>`__ for more details.
+
 ``for_js`` Method is Deprecated
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~