[cssom-1][css-conditional-3] We should mention that CSSMediaRule: CSSGroupingRule is superseded by css-conditional-3 in cssom

[Manishearth]

https://drafts.csswg.org/cssom/#the-cssmediarule-interface
https://drafts.csswg.org/css-conditional/#the-cssmediarule-interface

CSSOM specifies that CSSMediaRule: CSSGroupingRule, but css-conditional-3 specifies that CSSMediaRule: CSSConditionRule and CSSConditionRule: CSSGroupingRule. We should explicitly specify that the hierarchy in css-conditional-3 supersedes the cssom one in both specs, or update cssom to have CSSConditionRule (perhaps as an empty interface that gets extended by css-conditional)