[css-contain-2] Clarify that flat-tree descendants of content-visibility skipped elements include pseudos

[vmpstr]

The content-visibility text that talks about skipping rendering, says that contents are not painted, which is

the flat tree descendants of the element, including both text and elements, or the replaced content of a replaced element

The flat tree definition doesn't mention whether it includes pseudo elements, and my understanding is that it doesn't.

We should amend the content-visibility spec to explicitly say that contents of an element include, among other things, pseudo elements originating from the element.

/cc @tabatkins

[tabatkins]

c-v should definitely hide pseudos, like any other child. The only question I have is if it's best to handle this as a one-off in the c-v spec, or if we should add some more general definitions (in Display?) that specify that pseudos are in the flattened element tree.