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

[R. David Murray]

R. David Murray added the comment:

Chris, if Nick is too busy to reply right now and you want to move this along, you could write some tests (not necessarily for inclusion in the test suite) to confirm that the doc you are adding is correct.  I don't know enough about generators to comment myself, I'd have to write tests :)

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nosy: +r.david.murray

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<http://bugs.python.org/issue15355>
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