[css-borders-4] Specify how corner-shape affects outline

[noamr]

Follow up to #11610 (comment).
Outlines are "special" because they create a path that's external to the original border, more like a stroke.
A concave corner (e.g. a star of scoop shape) with a big outline might end up with very long miters.

Note that currently outline rendering is specified in pretty vague terms.

Proposing (based on suggestion from @smfr) to advise implementations to render a round miter for concave corner-shaped outlines.

[tabatkins]

Agree that we should aggressively cap the ability of corners to extend too much in a concave outline. I'm fine with a round miter, tho I don't think that's expressible in SVG right now? (stroke-linecap only affects the ends of segments.) I think that's just a lack in SVG tho; capping the mitered edges with as if stroke-linecap:arc applied to it sounds fine to me.

[fsoder]

I think the property you're looking for is stroke-linejoin (https://svgwg.org/svg2-draft/painting.html#LineJoin)