[selectors-4][editorial] Clarify when multiple scoping roots are provided

[cdoublev]

§ 8.4. The Reference Element Pseudo-class: :scope defines that it represents one or more scoping roots, and what :scope represents when zero or one scoping root is provided, but not when more than one scoping roots are provided. I hope the proposed change is obvious.

§ 17.5. Match a Selector Against a Tree defines that it takes one or more scoping roots but later at its step 2, it checks if scoping root were provided, which seems to be an oversight (singular subject, plural conjugation).

[cdoublev]

§ 8.4. The Reference Element Pseudo-class: :scope defines that it represents one or more scoping roots [...]

This is incorrect: :scope can be matched with respect to one or more scoping roots and can only match one of them. I pushed a commit to fix this mistake.