DDSXTY14 — Clarify accessing discriminator of a DynamicData union

// see pgh 1 of 7.2.2.4.4.3 for use of "discriminator" as a magic string
const MemberId id_disc = dyndata->get_member_id_by_name("discriminator"); 
if (id_disc == MEMBER_ID_INVALID) { some error happened }
// based on union_type_descriptor.discriminator_type->get_kind(), determine the type of the discriminator
// in this example, let's say it's an IDL long
int val;
dyndata->get_int32_value(val, id_disc);