[docs] [issue12245] Document the meaning of FLT_ROUNDS constants for sys.float_info

[Ezio Melotti]

Ezio Melotti <ezio.melotti at gmail.com> added the comment:

LGTM

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nosy: +ezio.melotti
stage:  -> commit review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12245>
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