[3.10] gh-93738: Documentation C syntax (:c:type:`PyUnicodeObject*` -> :c:expr:`PyUnicodeObject*`) (GH-97783) (#97860)

:c:type:`PyUnicodeObject*` -> :c:expr:`PyUnicodeObject*`
(cherry picked from commit 898834e27b82bd1f3532b6448a862a7a9cdeff66)

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