[3.12] gh-110893: Improve the documentation for __future__ module (GH-114642) (#114702)

gh-110893: Improve the documentation for __future__ module (GH-114642)

nedbat took issue with the phrasing "real module". I'm actually fine
with that phrasing, but I do think the `__future__` page should be clear
about the way in which the `__future__` module is special. (Yes, there
was a footnote linking to the future statements part of the reference,
but there should be upfront discussion).

I'm sympathetic to nedbat's claim that no one really cares about
`__future__._Feature`, so I've moved the interesting table up to the
top.
(cherry picked from commit 3b86891fd69093b60141300862f278614ba80613)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>