Move task shell description per draft comment

[robander]

The task element ref topic contained a high level overview of the 2 task shells, and a draft comment suggesting those should be in the arch spec. This update

• Moves them into the task overview topic in the arch spec, where they fit with the 2 topics that give a deep overview of the two shells
• Link to that topic from both task and taskbody usage info
• Includes a bit of cleanup in the relationship table map, as I'd briefly considered adding links between these topics