Docs: Improve the Mimir microservices mode explanation

[MichaelThamm]

Is your documentation request related to a feature? If so, which one?

Mimir microservices mode: make read requests to Mimir without the write microservice.

What is the solution that you would like or the expected outcome?

From the microservices mode documentation:

• https://grafana.com/docs/mimir/latest/references/architecture/deployment-modes/

It seems like a user would be able to make read requests to Mimir even if the write microservice goes down:

This would make sense because of the "microservice" mode.

What did you think would happen?

However, with some testing we noticed that, if the write microservice (distributor and ingester) becomes unavailable, then we cannot query mimir at the /prometheus/api/v1/query API due to:

expanding series: empty ring

Is this even a supported feature i.e., querying while write is down?

What was your environment?

TL;DR, running on Juju (backed by Canonical Kubernetes) with Mimir 2.13.0

Related issue:

• Too many unhealthy instances in the ring error when querying Mimir HA with all units green except for write ones canonical/mimir-worker-k8s-operator#120 (comment)

Any additional context to share?

No response