[3.10] gh-93738: Documentation C syntax (:c:type:`Py_UNICODE*` -> :c:expr:`Py_UNICODE*`) (GH-97784) (#97858)

:c:type:`Py_UNICODE*` -> :c:expr:`Py_UNICODE*`
(cherry picked from commit a081cae2a2cd1248ad067c3f7dc218ea7e3d203a)

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