Improve documentation for Real

[Henning Thielemann]

On Wed, 23 Dec 2020, David Feuer wrote:

> The Real class has one method:
> -- | the rational equivalent of its real argument with full precision
> 
> toRational :: a -> Rational
> 
> This is ... pretty weird. What does "full precision" mean? For integral and floating point types, it's fine. It's
> not at all meaningful for
> 
> 1. Computable reals
> 2. Real algebraic numbers
> 3. Real numbers expressible in radicals
> 4. Rational numbers augmented with some extra numbers like pi
> 5. Geometrically constructable reals
> 6. Etc.

They cannot have Real instances, then. Right?