[3.13] gh-122450: Expand documentation for Rational and Fraction (#136800)

[skirpichev]

(cherry picked from commit b266fbc)

• Issue: denominator of Fraction is positive, which should be documented #122450

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📚 Documentation preview 📚: https://cpython-previews--137367.org.readthedocs.build/

[skirpichev]

For some reasons witch wasn't helpful here and I ended with:

$ cherry_picker --continue
🐍 🍒 ⛏
Failed to amend the commit message ☹
fatal: You are in the middle of a cherry-pick -- cannot amend.

Backport PR URL:
https://github.com/python/cpython/compare/3.13...skirpichev:backport-b266fbc-3.13?expand=1
Error checking out the branch 'main'.
error: Your local changes to the following files would be overwritten by checkout:
        Doc/library/fractions.rst
Please commit your changes or stash them before you switch branches.
Aborting

branch backport-b266fbc-3.13 NOT deleted.

Backport PR:

[3.13] gh-122450: Expand documentation for ``Rational`` and ``Fraction`` (GH-136800)
(cherry picked from commit b266fbc9ecd25d08ab572ecb91913e86b6ddc75e)

Co-authored-by: Sergey B Kirpichev <[email protected]>
Co-authored-by: Adam Turner <[email protected]>
Co-authored-by: Serhiy Storchaka <[email protected]>

and with an empty pr (added files weren't commited).