[docs] [issue27644] Expand documentation about type aliases and NewType in the typing module

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

> Should I edit the patch to include a sentence or two describing this
> distinction, just remove the word "almost", or do something else?

I think the best way is to remove the "almost" and briefly explain that you cannot subclass types returned by NewType.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27644>
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