Expanding Data.Foldable documentation with overview prose (pre-MR feedback requested)

[Viktor Dukhovni]

On Sun, Dec 27, 2020 at 05:31:31AM -0500, Viktor Dukhovni wrote:

> > Otherwise: very well written.
> 
> Thanks, while reworking it, some things gone mangled, and I haven't
> finished the proofreading yet.  I'll fix these pronto.

Done.  I'd be really great if someone were interested in stepping
forward to take turns iterating on the document once or twice before
we posting an MR on Gitlab.  But if nobody has the cycles, or what
I has is deemed close enogh as-is, then I'll just proof read some
more and file an MR, probably before the year is out.

With a bit of luck (for this MR), given the delay in GHC 9.0, perhaps it
could even make it into the GHC 9.0 release.

-- 
    Viktor.