i32 -> u32 conversion described as "no-op"

[jgarvin]

From here: https://doc.rust-lang.org/nomicon/casts.html

casting between two integers of the same size (e.g. i32 -> u32) is a no-op

A signed integer can contain values an unsigned integer cannot, so even if this is true at the instruction level it seems misleading. Should maybe mention if Rust assumes 2s-complement representation?