[css-display] Clarify inline-level content can be boxes or text **runs**

[Loirooriol]

CSS Display defines inline-level as

Content that participates in inline layout. Specifically, inline-level boxes and text.

But "text" is an ambiguous term. I think it should be "text runs" instead. This is part of what I meant in #1186, back then text didn't live in the box tree, it was just a paint-time effect. But now we have this "text run" concept, so better use it.