add guide with extraArgs

Author: relusc

docs/guides/configure_memcached.markdown

@@ -17,9 +17,11 @@ has_toc: false
 ---
 # Configuring memcached as the caching store
 
-If you want to use [memcached](https://memcached.org/) as a caching store, you first have to deploy it separately (e.g. via separate helm chart). The existing deployments can be referenced in the `values.yaml` so that they are used as caching stores.
+If you want to use [memcached](https://memcached.org/) as a caching store, you first have to deploy it separately (e.g. via a dedicated helm chart). The existing deployments can be referenced by two approaches.
 
-Here's how to configure the memcached's via the values:
+## Referencing memcached stores via values
+
+Here's how to configure preexisting memcached deployment with their dedicated `memcached-*` sections in the chart values:
 
 {% raw %}
 ```yaml
@@ -41,3 +43,25 @@ memcached-blocks-metadata:
   containerPort: 11211
 ```
 {% endraw %}
+
+## Referencing via `extraArgs`
+
+You can also omit the `memcached-*` sections entirely and use the `extraArgs` of each cortex component to reference an existing memcached deployment.
+
+Here's an (incomplete) example:
+
+{% raw %}
+```yaml
+query_frontend:
+  extraArgs:
+    frontend.memcached.addresses: dns+memcached-frontend.memcached.svc.cluster.local:11211
+compactor:
+  extraArgs:
+    blocks-storage.bucket-store.index-cache.backend: memcached
+    blocks-storage.bucket-store.index-cache.memcached.addresses: dns+memcached-blocks-index.memcached.svc.cluster.local:11211
+ingester:
+  extraArgs:
+    blocks-storage.bucket-store.metadata-cache.backend: memcached
+    blocks-storage.bucket-store.metadata-cache.memcached.addresses: dns+memcached-blocks-metadata.memcached.svc.cluster.local:11211
+```
+{% endraw %}