PEP-654: clarify that BaseExceptionGroup is the factory function that… (GH-2203)

pep-0654.rst

@@ -150,12 +150,14 @@ fields ``message`` and ``exceptions``. For example:
 ``ExceptionGroup('issues', [ValueError('bad value'), TypeError('bad type')])``.
 The difference between them is that ``ExceptionGroup`` can only wrap
 ``Exception`` subclasses while ``BaseExceptionGroup`` can wrap any
-``BaseException`` subclass. A factory method that inspects the nested
-exceptions and selects between ``ExceptionGroup`` and ``BaseExceptionGroup``
-makes the choice automatic. In the rest of the document, when we refer to
-an exception group, we mean either an ``ExceptionGroup`` or a
-``BaseExceptionGroup``. When it is necessary to make the distinction, we
-use the class name. For brevity, we will use ``ExceptionGroup`` in code
+``BaseException`` subclass. The ``BaseExceptionGroup`` constructor
+inspects the nested exceptions and if they are all ``Exception`` subclasses,
+it returns an ``ExceptionGroup`` rather than a ``BaseExceptionGroup``. The
+``ExceptionGroup`` constructor raises a ``TypeError`` if any of the nested
+exceptions is not an ``Exception`` instance.  In the rest of the document,
+when we refer to an exception group, we mean either an ``ExceptionGroup``
+or a ``BaseExceptionGroup``. When it is necessary to make the distinction,
+we use the class name. For brevity, we will use ``ExceptionGroup`` in code
 examples that are relevant to both.
 
 Since an exception group can be nested, it represents a tree of exceptions,