bpo-45584: Clarify `math.trunc` documentation (GH-29183)

While floor/ceil 's documentation are very precise, `truncate` was not explained. I actually had to search online to understand the difference between `truncate` and `floor` (admittedly, once I remembered that numbers are signed, and that floating numbers actually uses a bit for negation symbol instead of two complement, it became obvious)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Éric Araujo <merwok@netwok.org>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>