DOCSP-38227 namespace filter restriction (#281)

Author: kyuan-mongodb

source/core/backup-preparations.txt

@@ -444,7 +444,11 @@ Namespaces Filter
 ~~~~~~~~~~~~~~~~~
 
 The namespaces filter lets you specify which databases and collections
-to back up. You create either a :guilabel:`Blacklist` of those to
+to back up. You can apply a namespace filter to any database except
+``admin`` and ``local`` and any collection that doesn't start with
+``system``. 
+
+You create either a :guilabel:`Blacklist` of those to
 exclude or a :guilabel:`Whitelist` of those to include. You make your
 selections when starting a backup and can later edit them as needed. If
 you change the filter in a way that adds data to your backup, a resync