Label collision described in 4.7 should be reflected in 3.1

[azaroth42]

We currently say in 3.1.: the value of the property MUST be a JSON object, and that an Annotation Collection (etc) SHOULD have the label property.

And in 4.7 we say: properties on Annotation based resources use the context from the Web Annotation Data Model, whereas properties on classes defined by this specification use the IIIF Presentation API context’s definition.

This is normatively inconsistent

[azaroth42]

Subsumed into #1834, but leave open to make sure the particular sections are updated.

[zimeon]

Closing as notes added into #1834