What makes an SDK "a complete package" ?

I have been working to develop SDKs around WhatsApp API products and very recently launched one i.e., https://www.producthunt.com/prod..., and also working on another one for the same aim in golang (because I needed it for one of our upcoming product). But I wonder what makes a SDK a complete package, what all necessities an SDK should have to make it a complete bundle to ship a good level application ?