bpo-26701: Add documentation for __trunc__ (GH-6049)

`int` fails back to `__trunc__` is `__int__` isn't defined, so cover
that in the docs.
(cherry picked from commit 308eab979d153f1ab934383dc08bc4546ced8b6c)

Co-authored-by: Eric Appelt <eric.appelt@gmail.com>
3 files changed