Add some documentation notes.

Author: ZeroIntensity

Doc/library/sys.rst

@@ -855,6 +855,11 @@ always available.
    reflect the actual number of references.  Consequently, do not rely
    on the returned value to be accurate, other than a value of 0 or 1.
 
+   .. impl-detail::
+
+      :term:`immortal` objects with a large reference count can be identified
+      with :func:`_is_immortal`.
+
    .. versionchanged:: 3.12
       Immortal objects have very large refcounts that do not match
       the actual number of references to the object.

Doc/whatsnew/3.14.rst

@@ -611,6 +611,9 @@ sys
   which only exists in specialized builds of Python, may now return objects
   from other interpreters than the one it's called in.
 
+* Add :func:`sys._is_immortal` for determining if an object is :term:`immortal`.
+  (Contributed by Peter Bierma in :gh:`128509`.)
+
 sys.monitoring
 --------------