[3.13] gh-70870: Clarify dual usage of 'free variable' (GH-122545) (#125088)

[3.13] gh-70870: Clarify dual usage of 'free variable' (GH-122545) (#125088)

The term "free variable" has unfortunately become genuinely
ambiguous over the years (presumably due to the names of
some relevant code object instance attributes).

While we can't eliminate that ambiguity at this late date, we can
at least alert people to the potential ambiguity by describing
both the formal meaning of the term and the common
alternative use as a direct synonym for "closure variable".

---------

(cherry picked from commit 27390990fa9306e2a797a4eb2bd83c5bfc7cb186)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>