Clarify aria-busy remediation message

[WilcoFiers]

We had some confusion about this message:

Element has no aria-busy="true" attribute

This doesn't explain when aria-busy would be appropriate. I think the following would be better:

Element uses aria-busy="true" while showing a loader

This required a small change to lib/checks/aria/aria-busy.json.

[straker]

To expand on this issue, we may not want to show the aria-busy remediation message when there are unallowed roles. Otherwise the remediation message suggests that adding aria-busy will fix the issue when it won't.

"Fix any of the following:
  Element has children which are not allowed: button
  Element has no aria-busy="true" attribute"