[issue31914] Document Pool.(star)map return type

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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keywords: +patch
nosy: +miss-islington
nosy_count: 4.0 -> 5.0
pull_requests: +28411
stage: backport needed -> patch review
pull_request: https://github.com/python/cpython/pull/30191

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31914>
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