[docs] [issue31304] Update doc for starmap_async error_back kwarg

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado <pablogsal at gmail.com>:


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keywords: +patch
pull_requests: +4137
stage: needs patch -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31304>
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