[3.12] gh-101699: Explain using Match.expand with \g<0> (GH-101701) (GH-115583)

[3.12] gh-101699: Explain using Match.expand with \g<0> (GH-101701) (GH-115583)

Update documentation for re library to explain that a backreference `\g<0>` is
expanded to the entire string when using Match.expand().
Note that numeric backreferences to group 0 (`\0`) are not supported.

(cherry picked from commit d2d78088530433f475d9304104bbc0dac2536edd)

Co-authored-by: Stevoisiak <S.Vascellaro@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>