[docs] [issue25735] math.factorial doc should mention integer return type

[miss-islington]

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


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pull_requests: +13588
pull_request: https://github.com/python/cpython/pull/13702

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue25735>
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