DDSXTY14 — Clarify how "must_understand" and "optional" members are designated

Section 7.2.2.4.4.4.6 "Members That Must Be Understood by Consumers" and 7.2.2.4.4.4.7 "Optional Members" talk about the concept of must understand members and optional members. Later sections 7.3.1 and 7.3.2 talk about representing types using IDL and XML. However these sections do not clearly say how the designation ties to those type representations.

The behavior can be deduced from the definition of certain annotations (like @optional) and their defaults. However there is still room for ambiguities.

Given this is fundamental to using the types It would be better if these specification was more explicit on how this designation happens using both IDL and XML.