Fix description of behaviour of an exception class in 'from' clause (GH-24303) (GH-25341)

(cherry picked from commit 79650d0) Co-authored-by: Mark Dickinson <mdickinson@enthought.com> Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

Author: miss-islington

Doc/reference/simple_stmts.rst

@@ -591,10 +591,13 @@ instance, with its traceback set to its argument), like so::
  __context__ (exception attribute)
 
 The ``from`` clause is used for exception chaining: if given, the second
-*expression* must be another exception class or instance, which will then be
-attached to the raised exception as the :attr:`__cause__` attribute (which is
-writable). If the raised exception is not handled, both exceptions will be
-printed::
+*expression* must be another exception class or instance. If the second
+expression is an exception instance, it will be attached to the raised
+exception as the :attr:`__cause__` attribute (which is writable). If the
+expression is an exception class, the class will be instantiated and the
+resulting exception instance will be attached to the raised exception as the
+:attr:`__cause__` attribute. If the raised exception is not handled, both
+exceptions will be printed::
 
  >>> try:
  ... print(1 / 0)