The manual covers the language, not the library. When you open the manual there is link that says "Standard library". There is only so much you do for readers who don't read. ;-)
Back to your question: Unsigned integers in Nim have wrap around semantics on overflow because unsigned was introduced for C interop. As numbers on their own these are not very useful as they don't have a working minus operator for the important case `N-1` when `N` is 0.
