bpo-42669: Document that `except` rejects nested tuples (GH-23822)

In Python 2, it was possible to use `except` with a nested tuple, and occasionally natural.  For example, `zope.formlib.interfaces.InputErrors` is a tuple of several exception classes, and one might reasonably think to do something like this:

    try:
        self.getInputValue()
        return True
    except (InputErrors, SomethingElse):
        return False

As of Python 3.0, this raises `TypeError: catching classes that do not inherit from BaseException is not allowed` instead: one must instead either break it up into multiple `except` clauses or flatten the tuple.  However, the reference documentation was never updated to match this new restriction.  Make it clear that the definition is no longer recursive.

Automerge-Triggered-By: GH:ericvsmith

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