Mention virtual subclasses in the glossary entry for ABCs (#12256).

I added a link from the term “virtual subclass” to the glossary entry
for ABCs but this was not enough, now the glossary briefly defines
“virtual” and links to the abc module doc which contains more mentions
of virtual subclasses.

Doc/glossary.rst

@@ -30,7 +30,10 @@ Glossary
       Abstract base classes complement :term:`duck-typing` by
       providing a way to define interfaces when other techniques like
       :func:`hasattr` would be clumsy or subtly wrong (for example with
-      :ref:`magic methods <special-lookup>`). Python comes with many built-in ABCs for
+      :ref:`magic methods <special-lookup>`).  ABCs introduce virtual
+      subclasses, which are classes that don't inherit from a class but are
+      still recognized by :func:`isinstance` and :func:`issubclass`; see the
+      :mod:`abc` module documentation.  Python comes with many built-in ABCs for
       data structures (in the :mod:`collections` module), numbers (in the
       :mod:`numbers` module), streams (in the :mod:`io` module), import finders
       and loaders (in the :mod:`importlib.abc` module).  You can create your own