Docs: Clarify that reference counts aren't stable between versions

Author: ZeroIntensity

Doc/glossary.rst

@@ -1202,6 +1202,11 @@ Glossary
       :func:`sys.getrefcount` function to return the
       reference count for a particular object.
 
+      In :term:`CPython`, reference counts are not considered to be stable, well-defined
+      values for an object; the number of references to a Python object, and how that number
+      is affected by Python code, may be different between versions. Consequently, don't rely
+      on an object's reference count to be a value other than 0 or 1.
+
    regular package
       A traditional :term:`package`, such as a directory containing an
       ``__init__.py`` file.