Add filtering by group to todos documentation

doc/workflow/todos.md

@@ -109,6 +109,7 @@ There are four kinds of filters you can use on your Todos dashboard.
 | Filter  | Description |
 | ------- | ----------- |
 | Project | Filter by project |
+| Group   | Filter by group |
 | Author  | Filter by the author that triggered the Todo |
 | Type    | Filter by issue or merge request |
 | Action  | Filter by the action that triggered the Todo |