[3.11] Improve some C API documentation (GH-108768) (GH-108786)

* Express functions which take argument as a C string in terms of
  functions which take Python object.
* Use "note" directive for PyMapping_HasKey() and
  PyMapping_HasKeyString() notes.

(cherry picked from commit 6f97eeec222f81bd7ae836c149872a40b079e2a6)