doc Explain aria.utils.Dom methods

The jsdoc didn't clearly say what kind of values should be expected
fix #1377 loading overlay juddering on scroll in Safari Mobile

Since overlay implementation was using fixed positioning,
the user experience was not very good in Safari Mobile
(e.g. on ipad) because the browser doesn't do repaints
of fixed-positioned elements during scrolling.

The implementation of overlay was changed to use equivalent
absolute positioning instead.