Add a comment about not used but required config file

bin/update_role

@@ -5,6 +5,8 @@
 #
 # Update a role on a remote node
 #
+# Note that even though the file `~/.BUBBLE_DEFAULT_DOMAIN` is required, it's content is ignored when this is executed.
+#
 # Usage:
 #
 #     update_role domain role-dir