clarify cardinality of ProductList/Product

3.2 ProductList
The ProductList specifies a list of products and product parts from the print buyer’s point of view. Not more than one Product[@IsRoot="True"] SHOULD be specified unless the XJDF describes a gang job where multiple final products are produced on a common sheet. For more details on
product intent, see Chapter 4 Product Intent.

4.1 ProductList
The products or set of products that are processed during a given workstep MAY be specified in a ProductList, which describes a bill of materials. Multiple final products SHOULD NOT be specified in an XJDF that describes product intent only, e.g. in a customer order. Multiple end final products MAY be specified, e.g. when a press sheet of a gang job that contains multiple individual customer jobs is printed.

<Merge 3.2 ProductList into 4.1 Product>