Returns the information about the types allowed in this property.
In a simple case like the following, an element property only has one TypeRef that points to String and tag name "foo".
However, in a general case an element property can be heterogeneous, meaning you can put different types in it, each with a different tag name (and a few other settings.)
// list can contain String or Integer.
In this case this method returns a list of two TypeRefs.
Always non-null. Contains at least one entry. If isValueList()==true, there's always exactly one type.