Clarify required fields for `PropertyValue`

[nickevansuk]

Clarify required fields for PropertyValue in each context, e.g:

• propertyID is REQUIRED for orderItemIntakeFormResponse in 5.6.3 Additional Details capture
• name and description are REQUIRED for accessCode: so update to In keeping with the Schema.org definition of PropertyValue, the name property MUST be used for the name of the property, and the description property MUST be used for any human-readable access control code(s).

The spec should clearly reflect the following:

    {
      "accessCode": {
        "requiredFields": [
          "type",
          "name",
          "description"
        ]
      },
      "additionalProperty": {
        "requiredFields": [
          "type",
          "name",
          "value"
        ]
      },
      "orderItemIntakeFormResponse": {
        "requiredFields": [
          "type",
          "propertyID",
          "value"
        ]
      }
    }