Unite the documentation for qmath.h functions

Some functions in qmath.h was documented under QtMath, and some under
QtCore/qmath.h, the latter does not appear to lead to functional links
in the function list, so we should move them all to QtMath.

Task-number: QTBUG-37799
Change-Id: I3118c65ccbfb2401bdc324dbef2885d4e7d1c8f6
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>