Commit 17112084f87d7ccebf639068b85948190d52c6ba introduced a new type
GHC.Prim. However, I don't see any documentation for this type in the
GHC.Prim haddock docs, or anywhere else. We should add this.
A little poking around tells me that
compiler/prelude/primops.txt.pp is the place to do it. But, I'm not familiar with that file's syntax, nor am I 100% familiar with
Proxy#, so I don't think I'm the one to do it.