Final call: Clarify acc support and assumption for element-lang-valid

Hey folks,
Another final call, lasting for 1 week. It's a minor change in which I've
added a clarification to the assumption, and added an accessibility support
note:
https://github.com/act-rules/act-rules.github.io/pull/1133

If you have any concerns, please leave a review before next Friday.

Kind regards,

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R