Docs: Add `os.splice` flags argument

[aminalaee]

The docs for os.splice misses the flags argument but it is documented in posixmodule.c:

https://github.com/python/cpython/blob/64ab9f7d5c7cbe5ef997c7d841151e0e71e7f582/Modules/posixmodule.c#L11565-L11566

Ref: https://github.com/python/typeshed/pull/10771

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