Remove "Known Issues"

"Known Issues" is no longer needed.
Move Info to upper
Rename css-overflow-clipping to css-containment

via https://dvcs.w3.org/hg/csswg/rev/08b791cbbf81
Add "Scroll Snap Points Level 1"

via https://dvcs.w3.org/hg/csswg/rev/b3ff4e7c9b66