[issue17188] Document 'from None' in raise statement doc.

[Nick Coghlan]

Nick Coghlan added the comment:

As Terry notes, the various pieces of "from None" documentation should also have a nearby directive like:

.. versionchanged: 3.3

   :const:`None` permitted as ``Y`` in ``raise X from Y`` 

.. versionadded: 3.3

   The ``__suppress_context__`` attribute to suppress automatic display of the exception context

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