Update IEEE docs

[tom-bop]

https://github.com/GaloisInc/cereal/blob/master/src/Data/Serialize/IEEE754.hs#L11 says that you use a method with ST to cast between IEEE types, but that's been reverted: cereal again uses Foreign to cast.

Ideally, instead of removing the comment, you could explain why you switched back? The git history (7cfabb6) makes it clear the decision was made for a good reason:

When serializing/deserializing a lot of nested structures with the intermediate array causes significant GC slowdowns.

...but a newcomer (e.g. me) might wonder why we use something that may be slower than other methods (e.g. http://hackage.haskell.org/package/reinterpret-cast claims to see 5x speedup)