docs: Update install_enterprise_k8s.md with redis username and password (#7593)

Co-authored-by: caitlinwheeless <caitlin@humansignal.com>

Author: makseq

docs/source/guide/install_enterprise_k8s.md

@@ -259,7 +259,7 @@ Adjust the included defaults to reflect your environment and copy these into a n
 
 </div>
 
-## Optional: set up TLS for PostgreSQL
+## Optional: Set up TLS for PostgreSQL
 To configure Label Studio Enterprise to use TLS for end-client connections with PostgreSQL, do the following:
 
 1. Enable TLS for your PostgreSQL instance and save Root TLS certificate, client certificate and its key for the next steps.
@@ -286,7 +286,7 @@ global:
 
 4. Install or upgrade Label Studio Enterprise using Helm.
 
-## Optional: set up TLS for Redis
+## Optional: Set up TLS for Redis
 To configure Label Studio Enterprise to use TLS for end-client connections with Redis, do the following:
 
 1. Enable TLS for your Redis instance and save Root TLS certificate, client certificate and its key for the next steps.
@@ -313,6 +313,51 @@ global:
 
 4. Install or upgrade Label Studio Enterprise using Helm.
 
+
+## Optional: Set up username and password for Redis
+
+Use one of these options to set a password and a username for Redis:
+
+**1. Password via Kubernetes Secret**. Use this when:
+* You want to avoid embedding credentials in `values.yaml`
+* You already manage Secrets in your cluster
+* You need a simple auth without multiple Redis users and you don't have username
+
+```yaml
+global:
+  redisConfig:
+    host: "redis://redis.example.com:6379/1"
+    password:
+      secretName: "my-redis-secret"   # Kubernetes Secret name
+      secretKey: "redis-password"      # Key inside Secret
+```
+
+**2. Username + password in URL**. Use this when:
+* Redis v.7 or later, and with ACL-enabled users
+* You need a dedicated Redis user for permission scoping
+* You need a quick, throwaway setup or local testing
+
+```yaml
+global:
+  redisConfig:
+    host: "redis://myuser:mypassword@redis.example.com:6379/1"
+```
+
+ **3. Username in environment variables + password in secret**. Use this when:
+* Redis v.7 or later, and with ACL-enabled users
+* You want to keep the password secret but still specify a username
+
+```yaml
+global:
+  redisConfig:
+    host: "redis://redis.example.com:6379/1"
+    password:
+      secretName: "my-redis-secret"   # Kubernetes Secret name
+      secretKey: "redis-password"      # Key inside Secret
+  extraEnvironmentVars:
+    REDIS_USERNAME: "myuser"           # Injected into pod env
+```
+
 ## Use Helm to install Label Studio Enterprise on your Kubernetes cluster
 
 Use Helm to install Label Studio Enterprise on your Kubernetes cluster. Provide your custom resource definitions YAML file. Specify any environment variables that you need to set for your Label Studio Enterprise installation using the `--set` argument with the `helm install` command.