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

[Petri Lehtinen]

Petri Lehtinen added the comment:

The patch should add something to the "The from clause is used for exception..." paragraph, to state that None is also allowed.

It also adds things in a confusing order. I think the new example should go below other examples, and the changedversion and addedversion directives should be the last thing in the section.

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nosy: +petri.lehtinen
versions:  -Python 3.2

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