Add documentation for promise_test() and worker tests.
Add documentation for promise_test() and worker tests.

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