You can attach documentation to states and transitions. In this context, documentation is some text offering additional information to a human reader. In the model, it does not serve any functional purpose.
By default, a state’s rectangle shows the state’s behaviour, and a transition shows its expression alongside its arrow. You can instead display an objects’s documentation, i.e., the documentation of a state or transition:
In order to return back to the object’s normal view, select Toggle documentation again.
While the documentation is shown, you can modify it in the statechart editor. Double-click on the documentation text field to start editing it. Click outside the state or transition to quit editing.
The properties view of states and transition always shows both documentation and statechart language elements side by side in different compartments. This might be a more comfortable way of editing any of them.