I have also failed to find the documentation for this. I have put together a list of the states called by the different vCAC workflows hacking the workflow definition using vCO:
AddingDisks
BuildComplete
BuildFinished
BuildingMachine
Complete
CustomizeMachine
CustomizeOS
DeactivateMachine
Disposing
EpiRegister
Failed
FailedProvisioning
FinalizeProvisioning
Finalized
InitialPowerOn
InitializeProvisioning
InstallOS
InstallingOS
MachineActivated
MachineProvisioned
Off
On
PowerOn
SccmRegistration
SetupOS
SetupPxe
I have not tested this so feedback is welcome.