Tighten the definition of a named tuple.

Doc/glossary.rst

@@ -340,7 +340,7 @@ Glossary
       also :term:`immutable`.
 
    named tuple
-      Any tuple-like class whose indexable fields are also accessible with
+      Any tuple subclass whose indexable fields are also accessible with
       named attributes (for example, :func:`time.localtime` returns a
       tuple-like object where the *year* is accessible either with an
       index such as ``t[0]`` or with a named attribute like ``t.tm_year``).