Add docstring.

Author: Peter Bierma

Include/cpython/object.h

@@ -527,4 +527,9 @@ typedef int (*PyRefTracer)(PyObject *, PyRefTracerEvent event, void *);
 PyAPI_FUNC(int) PyRefTracer_SetTracer(PyRefTracer tracer, void *data);
 PyAPI_FUNC(PyRefTracer) PyRefTracer_GetTracer(void**);
 
+/* Enable PEP-703 deferred reference counting on the object.
+ *
+ * If this is called on a build with the GIL enabled, this
+ * function does nothing.
+ */
 PyAPI_FUNC(int) PyUnstable_Object_SetDeferredRefcount(PyObject *);