Missing testharness documentation

[gsnedders]

The following things exposed in testharness don't appear to be in the docs:

OptionalFeatureUnsupportedError
begin_shadow_realm_tests
fetch_tests_from_shadow_realm
fetch_tests_from_window
fetch_tests_from_worker

[Siddharthgup]

Hi @gsnedders ,Can you assign this task to me ?