Define "play button" in audio element content has text alternative [e7aa44]

[WilcoFiers]

The definition of "play button" is put into a note in applicability. That's not really how things are done anymore in ACT. This needs to be fixed before we can hand this rule off to the ACT TF.

https://act-rules.github.io/rules/e7aa44#applicability

[WilcoFiers]

PR #1661 is adding a definition for play button. Assigning to Carlos who is working on this PR.