Update users.asciidoc (elastic#20802)

Adding note of clarity as users will not see a proper indice name if they have not previously run the {beatname_uc} setup command.

libbeat/docs/security/users.asciidoc

@@ -81,7 +81,7 @@ endif::has_ml_jobs[]
 Omit any privileges that aren't relevant in your environment.
 +
 NOTE: These instructions assume that you are using the default name for
-{beatname_uc} indices. If you are using a custom name, modify the privileges to
+{beatname_uc} indices. If +{beat_default_index_prefix}-*+ is not listed, or you are using a custom name, enter it manually and modify the privileges to
 match your index naming pattern.
 
 . Assign the *setup role*, along with the following built-in roles, to users who