[docs] [issue12760] Add create mode to open()

[Charles-François Natali]

Charles-François Natali <neologix at free.fr> added the comment:

> Just a small note: FileExistsError is raised, not exactly OSError,
> when the file exists.

I've updated the doc accordingly.

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stage: patch review -> commit review

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<http://bugs.python.org/issue12760>
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