Add ::backdrop inheritance change info

[bramus]

(👋 Hi, Chrome DevRel here)

When ::backdrop was moved from the fullscreen spec to css-position-4, it was made tree-abiding, inheriting from its originating element.

CSSWG Resolution: https://github.com/w3c/csswg-drafts/issues/7845#issuecomment-1789844429

This PR adds this information.

This feature is available in all major engines. (I’m not sure on Safari’s specifics so I’ve marked it as preview)

[bramus]

Got the info from Safari: it’s included in 17.4 (source)