Call out what the placeholders are for.

docs/defaults/separators.md

@@ -8,6 +8,10 @@ Below is an example of a separator, which can be seen surrounded by a red square
 
 ![](images/separators.jpg)
 
+## Important Note:
+
+Plex does not support creating these empty collections natively through its UI, so they can behave strangely in some cases, primarily related to showing/hiding them in the main library view.  As a workaround, you can use a template variable [`placeholder_SOMETHING_id`] to specify a single movie/show to add to these collections so they have one thing in them and will then behave as expected.
+
 ## Separator Files
 
 These are all the files that contain a separator