XTCE14 — Update XUSP Command and Command Packet Pattern

In the XUSP appendix, section "Command and Command Packet Pattern" should be updated with current lessons learned.

One lesson learned is having both command and telemetry ultimately inherit from a root container CCSDSPacket can confuse the living daylights out of some. Provide explainer, reasonable work around.

And while this document isnt a user guide, perhaps it could add little bit out basic cmd construction ... just a sentence or two

Moved forward from issue XUSP11-133.