document negative impls

[ehuss]

Unless I'm blind, I cannot find any discussion of negative impls. It is in the grammar, and mentioned briefly in the special types page. I think there should be a section in the implementations page.

EDIT: Oh, I'm just silly. Negative impls are not stable. However, since they are a part of the stable grammar, it should probably have at least some minor callout.

[Bert-King]

Smiling without a word...