[Yhc] Yhc .hi format docs

[Neil Mitchell]

> > The only thing to remember is that .hi files are written manually for
> > a couple of things to bootstrap stuff.
>
> Is that to resolve cyclic module dependencies al la GHC, or for some
> other reason?

Partially, yes, to resolve cyclic dependancies, but thats not required
for the Prelude etc - only in user code.

The other reason is that some files, PreludeBuiltin, exist only as .hi
files - defining things like (->). I tried to break this dependance at
some point, but failed miserably - I never could figure out how name
resolution worked, and therefore bootstrap it with the required names.

Thanks

Neil