QHeaderView::setOffsetToSectionPosition documentation update

QHeaderView::setOffsetToSectionPosition documentation update

The documentation has used the word visualIndex for the argument
to this function. That is however very misleading since this
visualIndex is not the same as 'visualIndex' as we have in

visualIndex(int logicalIndex)
logicalIndex(int visualIndex)
or visualIndexAt(int position)

Beside the above a variable name has also been changed from
visualIndex to actualVisualIndex.

Change-Id: I43808adfd7c287a349474ab1e2656c417c8cf3e9
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>