Source 3 Non-Det: Improve documentation for amb operator

To aid individuals coming from Source 3, the docs should mention that `amb` is not intended to behave like ordinary Source functions.

Specifically, when we evaluate `amb(alpha(), bravo())`, the call is not evaluated "inside out".
The expression `bravo()` will only be evaluated upon backtracking.

Moreover, `amb` cannot be passed around as a value.