gh-110893: Improve the documentation for __future__ module (#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.

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