[selectors] Don't make requirements in notes

[annevk]

https://w3c.github.io/csswg-drafts/selectors/#invalid has a must-level requirement in a note.

[svgeesus]

@fantasai @tabatkins

[SelenIT]

This note seems more like a clarification of the general rule about any unsupported syntactic constructs which it refers to when applied to selectors, rather than a requirement, to me. But I suppose that "any at-rules, properties, ..., and other syntactic constructs" statement is clear and unambiguous enough without such clarification. Maybe this note can be simply dropped?

[annevk]

Why would it say "Consistent with" then? "As per" would be better. And then "must" ought to be "will" as it's a statement of fact, not a requirement (the requirement is elsewhere).