Better document the return type of the fileglob filter (#52801)

https://github.com/ansible/ansible/pull/44986

Fixes #33465

Thanks to DaveB93 for the suggestion.

lib/ansible/plugins/lookup/fileglob.py

@@ -20,6 +20,7 @@ DOCUMENTATION = """
       - Patterns are only supported on files, not directory/paths.
       - Matching is against local system files on the Ansible controller.
         To iterate a list of files on a remote node, use the M(find) module.
+      - Returns a string list of paths joined by commas, or an empty list if no files match. For a 'true list' pass C(wantlist=True) to the lookup.
 """
 
 EXAMPLES = """