[css-selectors][css-mediaqueries] Clarify ASCII case-insensitive

[fred-wang]

Follow-up of #4599 ; @tabatkins had suggested me to check other "case-insensitive" parts. I'm copying what I said in #4599 (comment)

• "All Selectors syntax is case-insensitive within the ASCII range (i.e. [a-z] and [A-Z] are equivalent), except for parts that are not under the control of Selectors." https://www.w3.org/TR/selectors-3/#casesens => For non-exception this is just ASCII-case insensitiveness
• "The matching of C against the element's language value is performed case-insensitively within the ASCII range. " https://www.w3.org/TR/selectors-3/#lang-pseudo => Same here.
• " Pseudo-class names are case-insensitive. " https://www.w3.org/TR/selectors-3/#pseudo-classes => I wonder if this can be restricted to ASCII case-insensitive?
• "CSS style sheets are generally case-insensitive, and this is also the case for media queries. " https://www.w3.org/TR/css3-mediaqueries/#syntax => Same here.

[tabatkins]

Yes, all of those are ASCII CI.

That said, they're also all in Selectors 3. I've gone ahead and re-reviewed all uses of the term "case-sensitive" and "case-insensitive" in Selectors 4 and fixed them to all link, but I'm not sure it's worth doing the same to Selectors 3. @fantasai, opinions?

[fred-wang]

@tabatkins OK, and I see this is also fixed in https://drafts.csswg.org/mediaqueries-3/ so lgtm. thanks.

[frivoal]

(adjusted the labels to tie this issue to the spec where it still needs to be addressed, rather than to those where this used to be a problem, but has since been clarified)