Final call: make visible text content definition objective

Hey folks,
I've updated the definition of "visible text content", to make it
objective, so that it can be used in the applicability of the visible label
in accessible name rule. The final call ends on August 26th. If you have
any feedback, leave a review on the pull request:

https://github.com/act-rules/act-rules.github.io/pull/1419

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