Rollup merge of #58242 - notriddle:patch-3, r=zackmdavis
Rollup merge of #58242 - notriddle:patch-3, r=zackmdavis

Document the one TyKind that isn't documented

This is especially confusing since the name `Foreign` and the name `extern type` are so different. I deduced that they're the same by consulting git-blame.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust