gh-68163: Correct conversion of Rational instances to float (GH-25619) (GH-96557)

* gh-68163: Correct conversion of Rational instances to float

Also document that numerator/denominator properties are instances of Integral.

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>
(cherry picked from commit 8464b754)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>