Add a note in the glossary.

Author: ZeroIntensity

Doc/glossary.rst

@@ -658,6 +658,8 @@ Glossary
       and therefore it is never deallocated while the interpreter is running.
       For example, :const:`True` and :const:`None` are immortal in CPython.
 
+      Immortal objects can be identified via :func:`sys._is_immortal`.
+
    immutable
       An object with a fixed value.  Immutable objects include numbers, strings and
       tuples.  Such an object cannot be altered.  A new object has to