[PATCH] Document arithmetic overflow semantics

[Robert Dewar]

> and clearly this is in incorrect in many cases, for example int->long
> conversion and indeed anything that involves integer operands of
> different sizes.

Sorry for confusing things, but yes, of course the interpretation of values
is as signed values where it matters, but for addition, subtraction and
multiplication it does not, and the proper way of representing these in the
tree to get tyhe right results is to use unsigned oeprations.