[css-ruby][css-display] Clarify that the `display: block ruby` wrapper is a block container

[Loirooriol]

https://drafts.csswg.org/css-display-3/#valdef-display-ruby

The element generates a ruby container box and establishes a ruby formatting context in addition to [...] generating a wrapper box of the appropriate outer display type

https://drafts.csswg.org/css-ruby-1/#block-ruby

If an element has an inner display type of ruby and an outer display type other than inline, then it generates two boxes: a principal box of the required outer display type type, and an inline-level ruby container.

So for display: block ruby, both specs say that the wrapper is block-level, but they don't say that it's a block container (that's the only possibility that makes sense to me).