[skip ci] Fix 'Ord' documentation inconsistency

Current documentation for the `Ord` typeclass is inconsistent. It
simultaneously mentions that:

> The 'Ord' class is used for totally ordered datatypes.

And:

> The Haskell Report defines no laws for 'Ord'. However, '<=' is
> customarily expected to implement a non-strict partial order […]

The Haskell report (both 98 and 2010 versions) mentions total ordering,
which implicitly does define laws. Moreover, `compare :: Ord a => a -> a
-> Ordering` and `data Ordering = LT | EQ | GT` imply that the order is
indeed total (there is no way to say that two elements are not
comparable). This MR fixes the Haddock comment, and adds a comparability
law to the list of suggested properties.