TaskPriority should gain CustomStringConvertible, and docs for properties

[ktoso]

https://forums.swift.org/t/print-and-make-sense-of-task-currentpriority/61870

Makes fair points:

1. print("current priority = (Task.currentPriority)")
   prints the raw value which is pretty ugly, we should print a nicer representation, End users don't know about the underlying numeric values.

2. We should probably document all the fields a bit more, they're all lacking any kind of docs.
   Just by looking at docs you'll have no idea that low == utility, or that high == userInitiated.

[ktoso]

rdar://103052361