[css-display][css-contain] Rename "Becoming a formatting context"

[Loirooriol]

CSS Display (or is it CSS Contain?) has this transformation called "Becoming a formatting context". I think the name is confusing because it makes a box establish a formatting context, not become one (that wouldn't make much sense). It should be renamed to something like "Becoming a formatting context root".

[frivoal]

This seems reasonable to me. This seems purely editorial on something that doesn't have traction yet, so I don't think I need a group resolution, and I'll just do it.