[css3-ui] prose improvement to directional nav- (issue 29)

Here is a proposed patch to resolve issue 29 of [css3-ui]

https://wiki.csswg.org/spec/css3-ui#issue-29

It applies the prose we had previously resolved to use, and clarifies that if no element matches the selector specified in the nav-* property, the user agent automatically determines which element to navigate the focus to.

The behaviour was arguably implied by the previous prose, and is interoperable (tested Presto and samsung’s Chromium fork).

 - Florian