docs: Update cost optimisation to include information about cleaning …

…up workflows and archiving (argoproj#4549)

Signed-off-by: Zach Brown <[email protected]>

docs/cost-optimisation.md

@@ -39,9 +39,11 @@ Consider:
 
 > Suitable for all.
 
-A workflow (and for that matter, any Kubernetes resource) will incur a cost as long as they exist in your cluster. 
+A workflow (and for that matter, any Kubernetes resource) will incur a cost as long as they exist in your cluster, even after they are no longer running.
 
-The workflow controller memory and CPU needs increase linearly with the number of pods and workflows you are currently running. 
+The workflow controller memory and CPU needs increase linearly with the number of pods and workflows you are currently running.
+
+You should delete workflows once they are no longer needed, or enable a [Workflow Archive](workflow-archive.md) and you can still view them after they are removed from Kubernetes.
 
 Limit the total number of workflows using:
 
@@ -65,8 +67,6 @@ spec:
 
 You can set these configurations globally using [Default Workflow Spec](default-workflow-specs.md).
 
-If you need to keep records historically, use the [Workflow Archive](workflow-archive.md).
-
 Changing these settings will not delete workflows that have already run. To list old workflows:
 
 ```