address final review

Author: kumaraditya303

Doc/c-api/object.rst

@@ -805,11 +805,12 @@ Object Protocol
 .. c:function:: int PyUnstable_SetImmortal(PyObject *op)
 
    Marks the object *op* :term:`immortal`. The argument should be uniquely referenced by
-   the calling thread.
+   the calling thread. This is intended to be used for reducing reference counting contention
+   in the :term:`free-threaded build` for objects which are shared across threads.
 
    This is a one-way process: objects can only be made immortal; they cannot be
    made mortal once again. Immortal objects do not participate in reference counting
-   and will never be garbage collected.
+   and will never be garbage collected. If the object is GC-tracked, it is untracked.
 
    This function is intended to be used soon after *op* is created, by the code that
    creates it, such as in the object's :c:member:`~PyTypeObject.tp_new` slot.