[css-sizing] Clarify "elements capable of being ResizeObserver targets"

From https://drafts.csswg.org/css-sizing-4/#last-remembered

Only elements capable of being ResizeObserver targets can have a last remembered size.

This is confusing because the Resize Observer spec seems to allow all elements. CSS Sizing is actually referring to pseudo-elements, but pseudo-elements aren't actually elements, e.g. a property with "Applies to: all elements" doesn't apply to pseudo-elements unless otherwise stated, like ::before and ::after explicitly do.

So I would instead say

Only actual elements capable can have a last remembered size, pseudo-elements can't.

Or if you want to keep handwaving pseudo-elements as elements, at least add a note

Only elements capable of being ResizeObserver targets can have a last remembered size.
Note: Therefore, pseudo-elements can't have a last remembered size.

Start at the CSS Sizing 4 #last-remembered definition and compare its cited Resize Observer target wording. Resolve how pseudo-elements should be described, then update the definition or add the proposed note; done when the text clearly states whether pseudo-elements can have a last remembered size.