Clarify: PATCH should allow changing type on nested complex type

We say that a PATCH request to an entity with a complex type in the payload also patches the nested complex type (i.e., it only affects the properties specified in the request).

We don't say whether or not the PATCH request can change the complex type of the nested property.

While changing the type of an entity should not be allowed (and is, in fact, explicitly prohibited) because entities define an instance, the type of a complex-typed property should be able to be changed in a PATCH request by specifying a new complex type (i.e., @odata.type) in the payload.