Fix documentation in idlharness.js.
Fix documentation in idlharness.js.

zqzhang/testharness.js,ayg/testharness.js,RatneshThakur/testharness.js,hallvors/testharness.js,bboyle/testharness.js,jyasskin/testharness.js,hallvors/testharness.js,jyasskin/testharness.js