review(ui-primitives): stop the size doc from enumerating native sizes
The parenthetical listed 14/16 and this branch added 20, but the set never
held to it: IconRightUpOutline14 defaults to 8, IconTreeCorner8x10 to 10,
and IconWarningOutline16 to 14. Drop the list rather than maintain one that
drifts, and retitle the icons test that carried the same generalization.