lambda.Start documentation does not mention lambda.Handler interface

[rittneje]

Currently, the documentation for lambda.Start states that its parameter needs to be a function matching one of the given signatures.

However, it seems it can also accept an instance of the lambda.Handler interface, as revealed by both the implementation and deprecation comment on lambda.StartHandler.

Please fix the documentation for lambda.Start accordingly.

I would also like to point out that deprecating lambda.StartHandler was a bad idea, because it means we no longer have any compile-time safety.