Comment request flow for 5xxServeStale
Comment request flow for 5xxServeStale

Although this loop is quite neat, it isn't trivial to read. So add some
comments describing the flow.

mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests