gh-70870: Clarify dual usage of 'free variable' (#122545)

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".

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Co-authored-by: Carol Willing <carolcode@willingconsulting.com>