Supports non visual reading: unknown

[gregoriopellegrino]

In the principles document we have a string to indicate that the information on supports non-visual reading is unkown: "Not known if readable in read aloud and braille" (ID nonvisual-reading-unknown).

In the techniques, however, we do not have this situation because we indicate "May not be fully readable in read aloud and braille" (ID nonvisual-reading-may-not-fully) in the case of missing metadata.