Display warning about copy for lazy ByteString more prominent

I had a huge memory-leak in my application because when reading the documentation for `BSL.ByteString` suggests that `copy` should allow the "parent" to be released for GC. I thought this was a bit suspicious, but went with it anyways and it turned out quite bad... So I checked the source:

```haskell
-- | /O(n)/ Make a copy of the 'ByteString' with its own storage.
-- This is mainly useful to allow the rest of the data pointed
-- to by the 'ByteString' to be garbage collected, for example
-- if a large string has been read in, and only a small part of it
-- is needed in the rest of the program.
copy :: ByteString -> ByteString
copy cs = foldrChunks (Chunk . S.copy) Empty cs
--TODO, we could coalese small blocks here
--FIXME: probably not strict enough, if we're doing this to avoid retaining
-- the parent blocks then we'd better copy strictly.
```

Important part here is the *FIXME* which does not show up in the docs but is critical in my opinion. Can we move this there?