Add helper text for filter field

addons/core/translations/form/en-us.yaml

@@ -117,3 +117,4 @@ callback_url:
   help: The URL to which the third-party provider sends the authentication response.  Paste this into the provider configuration.
 filter:
   label: Filter
+  help: A boolean expression defining a filter run against the provided information.

ui/admin/app/components/form/managed-group/index.hbs

@@ -47,8 +47,9 @@
 
   <form.input
     @value={{@model.attributes.filter}}
+    @helperText={{t 'form.filter.help'}}
     @label={{t 'form.filter.label'}}
     readonly={{true}}
     @disabled={{true}}
   />
-</Rose::Form>
+</Rose::Form>