[docs] [issue15355] generator docs should mention already-executing exception

[Meador Inge]

Meador Inge added the comment:

Hi Chris, it seems reasonable to me, but I would feel more comfortable if someone (like Nick) that is more familiar with the generator implementation can comment on this as well.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15355>
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