Improve definition of "disabled element"

[Jym77]

The definition was mimicking https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-fe-disabled rather than the more generic https://html.spec.whatwg.org/multipage/semantics-other.html#concept-element-disabled
As far as I follow, this resulted in option elements never being disabled in our sense (well or indirectly via the aria-disabled attribute) and therefore, labels of <option disabled> where applicable for the colour contrast rule.

This is changing the def to the more generic one, using the CSS selector directly (so it also applies to non-HTML elements).
Additionally, the assumption in that definition is copied in the rule with indication of the consequences if it does not hold.

(noticed the issue while working on #1455)

Closes issue(s):

• N/A

Need for Final Call:
This will require a 1 week Final Call (indirectly changing Applicability of a rule)

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[kasperisager]

Just a small editorial suggestion, looks good otherwise!

Good catches. Corrected.

[Jym77]

Final Call ends on November 24.

[Jym77]

Final Call has ended. Merging.