[3.12] GH-107678: Improve Unicode handling clarity in ``library/re.rst`` (GH-107679) (#113965)

GH-107678: Improve Unicode handling clarity in ``library/re.rst`` (GH-107679)
(cherry picked from commit c9b8a22f3404d59e2c4950715f8c29413a349b8e)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>