[Bug c/88662] Document trap representations of _Bool

[gonzalobg88 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88662

--- Comment #2 from gnzlbg <gonzalobg88 at gmail dot com> ---
> Because (unsigned int)b is undefined otherwise.

AFAICT this is only undefined behavior iff `b` has a trap representation.