Clarify distinction between `Awakeable` and `DurablePromise`

[uosis]

It is not clear from the docs what are the differences and intended use cases. As far as I could tell, DurablePromise is just an Awakeable with a static id and only available in workflows.

For example, this SignupWorkflow example seems like it could have just as well been done using Awakeable by passing its id() in addition to secret to the click handler.

Is DurablePromise just minor sugar over Awakeable? Or is there more to it?