XUSP11 — Update Section 6.2.2

Section 6.2.2 needs to reflect some lessons learned in using GovSat. In particular it puts forth a "pattern" to use for defining telemetry packets using some common definitions in XTCE.
However real life is often a little more complicated... some T&C systems supply CCSDS header info outside the database itself, others use conventions.
At least these two need to be mentioned.