Document panicking cases for integer division
Document panicking cases for integer division

The panic on division by zero is expected, but the panic on overflow
is somewhat surprising (since most arithmetic operations panic on
overflow only when `debug_assertions` is enabled). As a result, it's
helpful to document this behavior.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust