Discussion: expand documentation of amap for IArray

The current documentation for Data.Array.IArray.amap says what it does, but doesn't say anything about how it should (and shouldn't!) be used. Specifically, it does not implement amap/amap or amap/coerce rules, and it probably cannot do so safely if the IArray instance does not satisfy the appropriate lens-like laws. Thus the most appropriate way to use this function is probably to implement the Functor instance for an IArray type, using type-specific RULES. It's probably *not* such a hot idea to use this amap directly. I don't know if all that should be crammed, with explanation, into the Haddocks or if there's a better way to handle it. David