GH-65056: Improve the IP address' is_global/is_private documentation (GH-113186)

* GH-65056: Improve the IP address' is_global/is_private documentation

It wasn't clear what the semantics of is_global/is_private are and, when
one gets to the bottom of it, it's not quite so simple (hence the
exceptions listed).

Co-authored-by: Petr Viktorin <encukou@gmail.com>