[css-omview] update description of getBoxQuads API

[jogibear9988]

The getBoxQuads API is missing lot of its description.

Issue in Firefox:
https://bugzilla.mozilla.org/show_bug.cgi?id=1107559

Description
https://hacks.mozilla.org/2014/03/introducing-the-getboxquads-api/

[jogibear9988]

I've also created a polyfill for this API https://github.com/jogibear9988/getBoxQuadsPolyfill

But in the polyfill I also added an offset DOMQuad to the getBoxQuads API. I'd like to suggest to add this also. This is usefull in a designer application, if you want to draw something around an element with an offset. For example a selected rectangle, wich should be offset a few pixels

[jogibear9988]

And the polyfill does not work correctly for SVG and MathML cause of missing offsetLeft/Top, see: #10514