Clarify use of type-specific facets in a union type

[jstoiko]

The description of the union type makes no mention of how to apply facets and the restrictions thereof.

Proposed clarification

• Add that a union type MAY have any of its super types facets as long as all super types participating in that union accept those facets

Examples

types:
  Foo: number
  Bar: integer
  FooBar:
    type: Foo | Bar
    minimum: 1 # valid because both "Foo" (number) and "Bar" (integer) all accept "minimum"

types:
  Foo: number
  Bar: integer
  Qux: string
  FooBarQux:
    type: Foo | Bar | Qux
    minimum: 1 # invalid because "Qux" (string) does not accept the "minimum" facet

types:
  Foo: number
  Bar: integer
  Qux:
    type: string
    facets:
      minimum: number
  FooBarQux:
    type: Foo | Bar | Qux
    minimum: 1 # valid because "Qux" (string) has a user-defined facet "minimum"