Document Enum choice restriction

[treeowl]

If fromEnum can produce negative numbers (e.g., if you try to make a BitSet Int), then everything breaks badly (GMP exceptions). As far as I can tell, this is not documented.

[superbobry]

Good catch, this is indeed undocumented.

I wonder if we can use another Enum-like class with non-negative fromEnum function? If not, I guess all we can do is document it and add a check to relevant functions.

[treeowl]

I discovered it by mistake when I tried to make a Data.BitSet.Generic.BitSet Int Word16 and accidentally made a Data.BitSet.Dynamic.BitSet Int.