Add docs for directly addressed todos

doc/workflow/todos.md

@@ -32,6 +32,29 @@ A Todo appears in your Todos dashboard when:
 
 >**Note:** Commenting on a commit will _not_ trigger a Todo.
 
+### Directly addressed Todos
+
+> [Introduced][ce-7926] in GitLab 9.0.
+
+If you are mentioned at the start of a line, the todo you receive will be listed
+as 'directly addressed'. For instance, in this comment:
+
+```markdown
+@alice What do you think? cc: @bob
+
+- @carol can you please have a look?
+
+>>>
+@dan what do you think?
+>>>
+
+@erin @frank thank you!
+```
+
+The people receiving directly addressed todos are `@alice`, `@erin`, and
+`@frank`. Directly addressed todos only differ from mention todos in their type,
+for filtering; otherwise, they appear as normal.
+
 ### Manually creating a Todo
 
 You can also add an issue or merge request to your Todos dashboard by clicking
@@ -85,8 +108,9 @@ There are four kinds of filters you can use on your Todos dashboard.
 | Project | Filter by project |
 | 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 (Assigned or Mentioned)|
+| Action  | Filter by the action that triggered the Todo |
 
 You can also filter by more than one of these at the same time.
 
 [ce-2817]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2817
+[ce-7926]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7926