Documentation improvements

The "Generic NFData deriving" documentation section shows how to derive instances of both `NFData` and `NFData1` for `Foo`, but it doesn't say anything about when generic instances need `NFData1` (the `NFData1` documentation doesn't help either). As this is in the `NFData` class documentation, for people like me who first encountered `NFData` through a package that depends on it (Criterion in my case), this gives the impression that there must be a mysterious technicality that requires you to derive `NFData1` for types with a single type parameter whenever you need an `NFData` instance. This is what I thought until I experimented and found that `deepseq` works fine on your `Foo` example without the `NFData1` instance. E.g. in GHCi 8.10.7:

```haskell
λ: data Foo a = Foo a String deriving(Generic,NFData)
λ: (Foo undefined "Hello" :: Foo Int) `deepseq` 1
*** Exception: Prelude.undefined
```

I see from the source code that `rnf = rnf1` for functors like list, `Maybe` and `Either`, so I now wonder if `NFData1` and `NFData2` are just helper classes that are only useful when you're manually deriving `NFData` instances. Are there cases where `NFData` cannot be generically derived for unary/binary polymorphic types without `NFData1/2`? If not, can the derivation of `NFData1` be removed from the `Foo` examples, and can a note be added stating that generic instances do not require `NFData1/2`? Or if `NFData1/2` _does_ need to be generically derived sometimes, can someone add an explanation to the documentation?