We already allow the stop API to be called when the VM is in Error
state. This is to update the diagram in devref doc.
Change-Id: I792028dc6f851f66d42b852c27505c479eab4df7
Added diagrams to RST docs that show required vm_state / task_state
transitions for compute api operations.
(Set Sandy Walsh as author since he created the diagrams)
Change-Id: Ie6a9cd4b54321a98fea5429ee52eb781f57f31a4