For simple queries like this it's best to just contact the source, e.g. here is the source code for RegisterAutoWired:
public IRegistration<T> RegisterAutoWired<T>()
{
var serviceFactory = GenerateAutoWireFn<T>();
return this.Register(serviceFactory);
}
It generates an auto-wired factory over a Concrete implementation. An interface has no implementation, it needs to be a concrete class.
And the source code for RegisterAs:
public IRegistration<TAs> RegisterAs<T, TAs>() where T : TAs
{
return this.RegisterAutoWiredAs<T, TAs>();
}
Which is just a shorter alias you can use instead of RegisterAutoWiredAs.