Document frontend worker

[owen-d]

Trying to answer #1863 I realized we don't have docs relating to the frontend worker (used when we run the query-frontend+queriers in grpc mode)

We should document the frontend worker configs

// WorkerConfig is config for a worker.
type WorkerConfig struct {
	Address           string
	Parallelism       int
	DNSLookupDuration time.Duration

	GRPCClientConfig grpcclient.Config `yaml:"grpc_client_config"`
}

[adityacs]

@owen-d Also, this is little confusing https://github.com/cortexproject/cortex/blob/12b15b9b2ecb36029e8f4330b4e2c01e7f180619/pkg/querier/frontend/frontend.go#L51

In Loki context, we should say it should be downstream querier/Loki URL

[owen-d]

haha good point