[docs] [issue27740] Fix doc of Py_CompileStringExFlags

[Xiang Zhang]

Xiang Zhang added the comment:

> filename is going to be decoded inside of Py_CompileStringExFlags

Actually all I want to express is this, *filename* will be decoded inside. But now the wording looks to me is that *filename* is already a decoded string when passed. I don't want to document anything about PyUnicode_EncodeFSDefault. I should remove (:func:`os.fsdecode`). How do you think? I'm Okay if you think the current wording is correct.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27740>
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