[docs] [issue18985] Improve the documentation in fcntl module

[R. David Murray]

R. David Murray added the comment:

Vajrasky: what do you think of my version?  Is it clear enough?

I wonder if we want to document the constants someday.

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stage:  -> patch review
versions: +Python 2.7, Python 3.3

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<http://bugs.python.org/issue18985>
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