Well, yes, if you're going to invent a special typeclass for every protocol, then you can document any signature. But what purpose does it serve? Abs to me seems like a hack, not something we really wanted to capture with the type system.

Do you find (x : Abs[T]) -> T in any way clearer than what's currently written in the docs? Do we really want to move in that direction? And not to mention that "... returns something of the same type" is _still_ incorrect -- for example, for complex it returns float.