document overflow semantics of Num instances of Bit{1,2,s}

[jwaldmann]

There are Num instances for Bit1, Bit2, Bits. Semantics should be clarified: how do the handle overflow?

It seems

• Bit{1,2} silently ignore overflow (so Bit arithmetical semantics is "modulo 2^n")
• Bits does never overflow (the result bit length might be enlarged)

There is one other option (that I need in some applications):

• numbers of fixed bit width where overflow implies unsatisfiability