[3.14] gh-141004: Document PyWeakref_CheckRefExact (GH-141279) (GH-141290)

gh-141004: Document `PyWeakref_CheckRefExact` (GH-141279)
(cherry picked from commit 6015532)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: miss-islington

Doc/c-api/weakref.rst

@@ -19,7 +19,14 @@ as much as it can.
 
 .. c:function:: int PyWeakref_CheckRef(PyObject *ob)
 
-   Return non-zero if *ob* is a reference object.  This function always succeeds.
+   Return non-zero if *ob* is a reference object or a subclass of the reference
+   type.  This function always succeeds.
+
+
+.. c:function:: int PyWeakref_CheckRefExact(PyObject *ob)
+
+   Return non-zero if *ob* is a reference object, but not a subclass of the
+   reference type.  This function always succeeds.
 
 
 .. c:function:: int PyWeakref_CheckProxy(PyObject *ob)