lambda.Start documentation does not mention lambda.Handler interface

Currently, the documentation for [`lambda.Start`](https://pkg.go.dev/github.com/aws/aws-lambda-go/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`](https://pkg.go.dev/github.com/aws/aws-lambda-go/lambda#Handler) interface, as revealed by both the implementation and deprecation comment on [`lambda.StartHandler`](https://pkg.go.dev/github.com/aws/aws-lambda-go/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.