Document that specs can depend on arguments of enclosing functions

Today I learned that the meaning of specs of local bindings can depend on arguments of an enclosing function. For instance, in the following function from the test suite, arg0 is available to the spec of bar.

{-@ foo :: x:_ -> y:_ -> {v:Int | v = x + y} @-}
foo :: Int -> Int -> Int
foo arg0 = bar
  where
    {-@ bar :: x:_ -> {v:Int | v = x + arg0} @-}
    bar arg1 = arg0 + arg1

It doesn't work for other things than arguments though, e.g. z is not in scope in the following spec.

{-@ foo :: x:_ -> y:_ -> {v:Int | v = x + y} @-}
foo :: Int -> Int -> Int
foo arg0 = bar
  where
    z = arg0
    {-@ bar :: x:_ -> {v:Int | v = x + z} @-}
    bar arg1 = arg0 + arg1

We should document the fact that arguments of enclosing functions are in scope. This is also true for other local bindings. See the comments below.