Document how to handle a workflow cancellation

A workflow cancelled event is sent when a workflow has been abandoned by the workflow initiator or an administrator.

Abandoning a workflow leaves the document(s) the workflow is associated to in its current state, nothing is rolled back.

Currently, nothing is documented regarding this event and it is not clear to users that they could do something to possibly roll back changes made when a workflow is cancelled.

TODO

• Document that an event handler listening to the workflow cancelled event can help in that regard, an example can be found in https://connect.nuxeo.com/nuxeo/site/studio/ide?project=nuxeo-routing-default#@feature:cancelWorkflow.ops