Tone and style edits to pages

Make small changes to the work already done by engineering to bring
it closer to GitLab tone and style.

doc/user/clusters/applications.md

@@ -64,21 +64,35 @@ supported by GitLab before installing any of the applications.
 > - Introduced in GitLab 10.2 for project-level clusters.
 > - Introduced in GitLab 11.6 for group-level clusters.
 > - [Uses a local Tiller](https://gitlab.com/gitlab-org/gitlab/-/issues/209736) in GitLab 13.2 and later.
+> - [Uses Helm 3](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46267) for clusters created with GitLab 13.6 and later.
 
 [Helm](https://helm.sh/docs/) is a package manager for Kubernetes and is
 used to install the GitLab-managed apps. GitLab runs each `helm` command
 in a pod within the `gitlab-managed-apps` namespace inside the cluster.
 
-GitLab's integration uses Helm 2 with a local
-[Tiller](https://v2.helm.sh/docs/glossary/#tiller) server for managing
-applications. Prior to [GitLab 13.2](https://gitlab.com/gitlab-org/gitlab/-/issues/209736),
-GitLab used an in-cluster Tiller server in the `gitlab-managed-apps`
-namespace. This server can now be safely removed.
+- For clusters created on GitLab 13.6 and newer, GitLab uses Helm 3 to manage
+  applications.
+- For clusters created on versions of GitLab prior to 13.6, GitLab uses
+  Helm 2 with a local [Tiller](https://v2.helm.sh/docs/glossary/#tiller) server.
+  Prior to [GitLab 13.2](https://gitlab.com/gitlab-org/gitlab/-/issues/209736),
+  GitLab used an in-cluster Tiller server in the `gitlab-managed-apps`
+  namespace. You can safely remove this server after upgrading to GitLab 13.2
+  or newer.
 
 GitLab's Helm integration does not support installing applications behind a proxy,
 but a [workaround](../../topics/autodevops/index.md#install-applications-behind-a-proxy)
 is available.
 
+#### Upgrade a cluster to Helm 3
+
+GitLab does not currently offer a way to migrate existing application management
+on existing clusters from Helm 2 to Helm 3. To migrate a cluster to Helm 3:
+
+1. Uninstall all applications on your cluster.
+1. [Remove the cluster integration](../project/clusters/add_remove_clusters.md#removing-integration).
+1. [Re-add the cluster](../project/clusters/add_remove_clusters.md#existing-kubernetes-cluster) as
+   an existing cluster.
+
 ### cert-manager
 
 > Introduced in GitLab 11.6 for project- and group-level clusters.

doc/user/project/clusters/securing.md

@@ -42,7 +42,7 @@ Minimum requirements (depending on the GitLab Manage Application you want to ins
 
 NOTE: **Note:**
 These diagrams use the term _Kubernetes_ for simplicity. In practice, Sidekiq connects to a Helm
-Tiller daemon running in a pod in the cluster.
+command runner pod in the cluster.
 
 You install GitLab Managed Apps from the GitLab web interface with a one-click setup process. GitLab
 uses Sidekiq (a background processing service) to facilitate this.