docs: Updated documentation for customizing existing addons

Updated documentation for customizing existing addons

Signed-off-by: Deepak Devadathan <deepak.devadathan@oracle.com>

Author: ddevadat

docs/src/guide/cluster_addons.md

@@ -10,6 +10,8 @@ Cluster add-on removal (using the `cluster_addons_to_remove` variable) requires
 - the stand-alone cluster-autoscaler deployment, using the [extension module](./extensions_cluster_autoscaler.md)
 - the cluster-autoscaler add-on
 
+When customizing the configuration of an existing addon, use the flag `override_existing=true`. Default value is false if not specified.
+
 ## Example usage
 ```javascript
 {{#include ../../../examples/cluster-addons/vars-cluster-addons.auto.tfvars:4:}}