Proposal: Warn about ArrowLoop instance for Kleisli Arrows

[Paterson, Ross]

Heinrich Apfelmus wrote:
>Bas van Dijk wrote:
>> On 25 October 2011 14:47, Heinrich Apfelmus
>>> What exactly do I have
>>> to do to move this towards inclusion in the base
>>> libraries? Make a track ticket? And then?
>> [...]
>> Not to spoil anything but I saw Ross already made the change ;-)
>
> That's great, thanks! The procedure for Compiling GHC did look rather
> daunting to me. :)

I trimmed the warning a little.  I also added the laws for the classes while I was there.