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

[Charles-François Natali]

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

Nick suggested to call the new flag "exclusive create" in the doc (and explain in whatsnew that it's based C11 new 'x' flag).
Could someone please check the attached patch?
My wording sounds really clumsy, so I'd prefer if a native speaker could review it.

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nosy: +ncoghlan
Added file: http://bugs.python.org/file24195/x_flag.diff

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