Keep "virtual subclass" tied to ABCMeta.register(), as in the abc docs

Author: thejesh23

Doc/glossary.rst

@@ -1731,11 +1731,12 @@ Glossary
    virtual subclass
       A class that :func:`issubclass` and :func:`isinstance` treat as a
       subclass of an :term:`abstract base class` even though it does not
-      inherit from it.  A class becomes a virtual subclass when it is
-      registered with the ABC's :meth:`~abc.ABCMeta.register` method, or when
-      the ABC's :meth:`~abc.ABCMeta.__subclasshook__` recognizes it.  The ABC
-      does not appear in the virtual subclass's :term:`MRO`, and none of the
-      ABC's methods are inherited.
+      inherit from it.  A class is made a virtual subclass by registering
+      it with the ABC's :meth:`~abc.ABCMeta.register` method.  An ABC can
+      also recognize classes without registration by overriding
+      :meth:`~abc.ABCMeta.__subclasshook__`.  The ABC does not appear in
+      the virtual subclass's :term:`MRO`, and none of the ABC's methods are
+      inherited.
 
    walrus operator
       A light-hearted way to refer to the :ref:`assignment expression