docs: attribute the compiled failure cost to the fallback, not the double pass

The compiled fast path costs 6.8 ns of a failing compiled safeParse's 534.5, so
paying for it twice is 1.3% of the cost. What makes failures slow is the
fallback, which produces the errors and is not compiled. Compilation does speed
failures up a little (1.23x), so "does not" overstated it.