[docs] [issue18697] Unify arguments names in Unicode object C API documentation

[Windson Yang]

Windson Yang <wiwindson at outlook.com> added the comment:

I agreed with @Matheus, it would be better than the current implementation

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nosy: +Windson Yang
versions: +Python 3.6, Python 3.7 -Python 2.7, Python 3.4, Python 3.5

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<https://bugs.python.org/issue18697>
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