Remove outdated note on hit-testing and root element

[AmeliaBR]

The instructions for hit-testing say:

Note that the ‘svg’ element is not a graphics element, and in a Conforming SVG Stand-Alone File a outermost svg element will never be the target of pointer events, though events can bubble to this element. If a pointer event does not result in a positive hit-test on a graphics element, then it should evoke any user-agent-specific window behavior, such as a presenting a context menu or controls to allow zooming and panning of an SVG document fragment.

This text hasn't changed since SVG 1.1.

It doesn't match implementations, or the general rule used elsewhere that the root SVG behaves as a CSS layout box. CSS layout boxes catch clicks. (Demo interactive SVG that relies on this being true)

Related to #703; need to more comprehensively define what it means that the root SVG is a CSS layout box (and how that impacts non-CSS environments).