Clarify cluster deletion behavior

All of the cluster deletion APIs only
delete the linked cluster object in GitLab
and do not invoke the cleanup functions
that's available as an option through
the UIs.

This change adds sentences to all 3
APIs that delete clusters to clarify
their behavior w.r.t. underlying resources
within the connected Kubernetes clusters.

doc/api/group_clusters.md

@@ -315,7 +315,7 @@ Example response:
 ## Delete group cluster
-Deletes an existing group cluster.
+Deletes an existing group cluster. Does not remove existing resources within the connected Kubernetes cluster.
 ```plaintext
 DELETE /groups/:id/clusters/:cluster_id

doc/api/instance_clusters.md

@@ -290,7 +290,7 @@ Example response:
 ## Delete instance cluster
-Deletes an existing instance cluster.
+Deletes an existing instance cluster. Does not remove existing resources within the connected Kubernetes cluster.
 ```plaintext
 DELETE /admin/clusters/:cluster_id

doc/api/project_clusters.md

@@ -388,7 +388,7 @@ Example response:
 ## Delete project cluster
-Deletes an existing project cluster.
+Deletes an existing project cluster. Does not remove existing resources within the connected Kubernetes cluster.
 ```plaintext
 DELETE /projects/:id/clusters/:cluster_id