Update PROPERTY_HIGH_DEF_AUDIO API docs.

Make it more clear that the intention of the property is to convey whether
a call is high definition on the UI or not, and NOT to report the actual
audio codec in use.

Test: Build
Bug: 78550833
Change-Id: Id002cd1c9cc1d5aea6f8d9ab4e3d99956b7c42b2