Minimum number of properties not mentioned in the property reference

The [`mesh.primitive.attributes` schema](https://github.com/KhronosGroup/glTF/blob/6aceb05b00bc9b49333bc1e41f3069bf5917e1f7/specification/2.0/schema/mesh.primitive.schema.json#L12) defines

"minProperties": 1,

This is not mentioned in the [`mesh.primitive.attributes` property reference](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#_mesh_primitive_attributes).

(There might be further instances of this, just giving one example here)

(An aside: For arrays, there is `minItems`/`maxItems`, and this is summarized somewhat implicitly in the "array size suffixes" like `[1-*]`. For dictionaries, there is `minProperties`/`maxProperties`, and this is not handled by wetzel yet)