Doc fix for 'frexp' in arith.texi

It is the magnitude of the return value which lies
in [0.5, 1), not the return value itself.

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2013-05-28  Ben North  <ben@redfrontdoor.org>

	* manual/arith.texi (frexp): It is the magnitude of the return
	value which lies in [0.5, 1), not the return value itself.