Add documentation for AddOnDeploymentConfig namespace behavior (#522)

This commit adds documentation explaining how the agentInstallNamespace
field in AddOnDeploymentConfig interacts with the namespace defined in
AddOnTemplate manifests.

Key points documented:
- When AddOnDeploymentConfig is not used, the AddOnTemplate namespace is used
- When AddOnDeploymentConfig is used with agentInstallNamespace: "",
  the AddOnTemplate namespace is used
- When AddOnDeploymentConfig is used but agentInstallNamespace is not set,
  the default "open-cluster-management-agent-addon" is used
- When agentInstallNamespace is set to a specific value, that value is used

This addresses the behavior fix from open-cluster-management-io/ocm#1209

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: zhujian <[email protected]>
Co-authored-by: Claude <[email protected]>

Author: zhujian7

content/en/docs/developer-guides/addon.md

@@ -1344,6 +1344,32 @@ spec:
 
 This allows you to have different variable values for different clusters while using the same addon template.
 
+#### Namespace configuration with AddOnDeploymentConfig
+
+When using `AddOnDeploymentConfig` with addon templates, the addon agent installation namespace is determined as follows:
+
+* If `AddOnDeploymentConfig` is **not used**, the namespace of the manifest defined in the `AddOnTemplate` is used.
+* If `AddOnDeploymentConfig` is **used** but `agentInstallNamespace: ""` (empty string), the namespace from `AddOnTemplate` is used.
+* If `AddOnDeploymentConfig` is **used** but `agentInstallNamespace` is **not set**, the default namespace `open-cluster-management-agent-addon` is used.
+* If `agentInstallNamespace` is set to a specific namespace, that namespace is used.
+
+**Important:** To preserve a custom namespace from your `AddOnTemplate` when using `AddOnDeploymentConfig`, explicitly set `agentInstallNamespace: ""`.
+
+Example:
+
+```yaml
+apiVersion: addon.open-cluster-management.io/v1alpha1
+kind: AddOnDeploymentConfig
+metadata:
+  name: hello-template-deploy-config
+  namespace: open-cluster-management
+spec:
+  agentInstallNamespace: ""  # Use namespace from AddOnTemplate
+  customizedVariables:
+  - name: LOG_LEVEL
+    value: "2"
+```
+
 #### Variable naming and validation
 
 When defining customized variables in AddOnDeploymentConfig, please note:

content/en/docs/getting-started/installation/addon-management.md

@@ -207,6 +207,8 @@ spec:
     resource: addontemplates
 ```
 
+**Note on namespace configuration:** When using `AddOnDeploymentConfig` with addon templates, if you want to preserve the namespace defined in your `AddOnTemplate`, you must explicitly set `agentInstallNamespace: ""` in the `AddOnDeploymentConfig`. Otherwise, the default namespace `open-cluster-management-agent-addon` will be used. See [Namespace configuration with AddOnDeploymentConfig]({{< ref "docs/developer-guides/addon/#namespace-configuration-with-addondeploymentconfig" >}}) for details.
+
 ### Configurations per install strategy
 
 In `ClusterManagementAddOn`, `spec.installStrategy.placements[].configs` lists the
@@ -390,4 +392,4 @@ status:
     lastObservedGeneration: 1
     name: cluster1-addon-template
     resource: addontemplates
-```
+```