[selectors-4] mention the specificity rules for special pseudo-classes

[chriseppstein]

There are special rules for certain functional psuedo-classes called out here:
https://drafts.csswg.org/selectors-4/#specificity-rules

• :matches()
• :is()
• :nth-child(), :nth-last-child(), :nth-of-type(), or :nth-last-of-type()
• :not()

I think it would be very helpful if those special rules were noted (probably with a link) in the specific sections for each pseudo class. E.g. https://drafts.csswg.org/selectors-4/#matches-pseudo

[chriseppstein]

🎉