[css-display] Specify that some display-inside values are "prose-capable"

[tabatkins]

Some display values can handle prose inside themselves, some can't. Those that can't transform inline-level elements to block-level, but that's not always enough.

In particular, if you have a "ruby-base" inside a "flex", it'll generate a "ruby" box around itself, but then needs to additionally generate a "block" box around itself.

Alternately: define how "display: block-level ruby;" works.