[Call for Review] Editorials to "audio has alternative" rules

The below PR proposes some edits to the rule descriptions and test descriptions of the following rules.

* '`audio` element content is media alternative for text'
* '`audio` element content has transcript'
* '`audio` element content has text alternative'

Call for Review ends Thursday March 31st.

Best.


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Daniel Montalvo

Accessibility Education and Training Specialist
W3C/WAI