Clarify none and unknown are not valid features

[mattgarrish]

It doesn't make sense to require that features be specified but also allow these values, so this pull request adds a paragraph explaining that the values are not allowed to meet the reporting requirements.

Fixes #2537

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[madeleinerothberg]

Consider adding one more phrase to the end of the paragraph, so that it reads something like this:

Authors must indicate at least one feature to claim conformance to EPUB Accessibility 1.1.

[mattgarrish]

@clapierre @gregoriopellegrino @avneeshsingh @GeorgeKerscher are you guys okay with merging this and adding unknown to the schema.org vocabulary?

[GeorgeKerscher]

Hi, So I see the following:
Be aware that although the vocabulary for the accessibilityFeature property [a11y-discov-vocab] contains the values "none" and "unknown", these terms cannot be used to meet the reporting requirements for the property. Authors must indicate at least one feature.
Refer to The accessibilityFeature Property [a11y-discov-vocab] for more information about this property and its values.

I think this is fine.
/George

[mattgarrish]

Thanks George!

Authors must indicate at least one feature.

Per @madeleinerothberg's suggestion above, this last sentence now reads: "Authors must indicate at least one feature to claim conformance to EPUB Accessibility 1.1."

[gregoriopellegrino]

For non-native English readers, I would add (redundantly):

at least one feature (apart from none and unknown)

[gregoriopellegrino]

Thank you!

[mattgarrish]

I used slightly different wording to avoid repeating the terms again as I was looking at the vocabulary and some day we might want to look at listing the other values that aren't applicable to epubs, like taggedPDF.

[madeleinerothberg]

Looks good! I agree that the additional-additional phrase adds clarity.