Fix Diff documentation

[dsnet]

The description inaccurately describes the operation of Diff,
which is y - x, where a '+' prefix denotes elements added from y
and a '-' prefix denotes elements removed from x.

For example:

// Consider this call to Diff and its result.
                  x  y
cmp.Diff({b:2, c:3}, {a:1, b:2}) => {+a:1, b:2, -c:3}

// Consider the same in mathematical notation.
         y - x
{a:1, b:2} - {b:2, c:3} = {+a:1, b:2, -c:3}