SortRel is a little under documented

[westonpace]

1. What does SORT_DIRECTION_CLUSTERED mean?
2. Let's say I was sorting by strings, how would I specify a natural sort vs alphabetical sort?
3. Can we provide an example of comparison_function_reference? What kind of function is this? Why isn't this just an extension relation?
4. Are we content leaving the stability of the sort as a hint? It seems like the producer / planner might care about this.