Add documentation about todos and failed builds

doc/workflow/todos.md

@@ -24,9 +24,11 @@ you still have open.
 A Todo appears in your Todos dashboard when:
-- an issue or merge request is assigned to you
+- an issue or merge request is assigned to you,
 - you are `@mentioned` in an issue or merge request, be it the description of
-  the issue/merge request or in a comment
+  the issue/merge request or in a comment,
+- build in the CI pipeline running for your merge request failed, but this
+  build is not allowed to fail.
 >**Note:** Commenting on a commit will _not_ trigger a Todo.