Add new unknown states for Audio/Motion/Flashing Hazards

[clapierre]

There is a need to specify if a particular Hazard hasn't been considered if that type of content exists in a Publication.

For example if a publication has no Video content the author can clearly state noMotionSimulationHazard and noFlashingHazard

However if that same publication includes an audio clip which hasn't been "examined" to know if it has or doesn't have any potential sound hazards, the only option would be not to include that statement in the accessibility Metadata, instead of clearly pointing out that the author doesn't know if a hazard exists or not.

Recommendation as discussed in the 7/17/2023 Editors call is to add 3 new unknown values to the Hazard metadata

unknownFlashingHazard, unknownMotionSimulationHazard, and unknownSoundHazard

The following two documents should be updated https://w3c.github.io/a11y-discov-vocab/index.html and https://w3c.github.io/a11y-discov-vocab/crosswalk/index.html

[mattgarrish]

As an additional bit of clarification, this request for individual unknown properties came from the publishing community group. The lack of clarity about what constitutes a sound hazard leaves most publishers needing to say they don't know, either.

The call @clapierre references was for the guide for displaying the metadata, as we were discussing what to do if the publisher leaves out any sound hazard metadata. It would be better to provide the unknown option.

Since we already allow individual "no hazard" statements, it seems reasonable to add equivalent "unknown hazard" ones. I don't really view this as new metadata, so I'm fine with it.

[mattgarrish]

Closing with #93