(DOCSP-22756) Update Charts docs for Authenticated Embedding for Realm provider (#541)

* (DOCSP-22756) Update Charts docs for Authenticated Embedding for Realm provider

Author: zach-carr

source/filter-embedded-charts.txt

@@ -262,6 +262,16 @@ This filter ensures that viewers of an embedded chart see only their
 own data, which is useful when embedding charts with potentially
 sensitive information.
 
+.. note:: 
+
+   If you use a Realm Authentication Provider, ``context.token`` 
+   contains the Realm 
+   :atlas:`user object </app-services/authentication/user-objects/>` to 
+   filter. For example, if you enable 
+   :atlas:`Custom User Data </app-services/users/enable-custom-user-data/>` 
+   for Realm users, the user object is available in 
+   ``context.token.custom_data``.
+
 To inject a filter specific to each user, in the 
 :guilabel:`Authenticated` tab of the the :guilabel:`Embed Chart` 
 dialog, set the :guilabel:`Inject Filter Per User`