Link isinstance/issubclass to the ABC glossary entry (#12256)

Doc/library/functions.rst

@@ -624,7 +624,8 @@ are always available.  They are listed here in alphabetical order.
 .. function:: isinstance(object, classinfo)
 
    Return true if the *object* argument is an instance of the *classinfo*
-   argument, or of a (direct or indirect) subclass thereof.  If *object* is not
+   argument, or of a (direct, indirect or :term:`virtual <abstract base
+   class>`) subclass thereof.  If *object* is not
    an object of the given type, the function always returns false.  If
    *classinfo* is not a class (type object), it may be a tuple of type objects,
    or may recursively contain other such tuples (other sequence types are not
@@ -634,7 +635,8 @@ are always available.  They are listed here in alphabetical order.
 
 .. function:: issubclass(class, classinfo)
 
-   Return true if *class* is a subclass (direct or indirect) of *classinfo*.  A
+   Return true if *class* is a subclass (direct, indirect or :term:`virtual
+   <abstract base class>`) of *classinfo*.  A
    class is considered a subclass of itself. *classinfo* may be a tuple of class
    objects, in which case every entry in *classinfo* will be checked. In any other
    case, a :exc:`TypeError` exception is raised.