Documentation improvements for the URec instance

This fixes a handful of minor issues with the `NFData` instance for
`URec`:

* It had a `@since` annotation without Haddock formatting.
* It did not mention that it was only available on `base-4.9` or
  later.
* The CPP referred to `__GLASGOW_HASKELL__`, not
  `MIN_VERSION_base(4,9,0)` like it should have.

This patch fixes all of these problems.