[3.12] gh-110527: Improve PySet_Clear docs (GH-110528) (#110928)

gh-110527: Improve `PySet_Clear` docs (GH-110528)
(cherry picked from commit bfc1cd8)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Doc/c-api/set.rst

@@ -163,4 +163,6 @@ subtypes but not for instances of :class:`frozenset` or its subtypes.
 
 .. c:function:: int PySet_Clear(PyObject *set)
 
-   Empty an existing set of all elements.
+   Empty an existing set of all elements. Return ``0`` on
+   success. Return ``-1`` and raise :exc:`SystemError` if *set* is not an instance of
+   :class:`set` or its subtype.