Add details into docs about semantics of 'on-XXX' clauses.
Add details into docs about semantics of 'on-XXX' clauses.

Change-Id: I003b7940ed8e2eee9d7dbd61261a6585964df029
Closes-Bug: #1642908

openstack/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral