Update MHC documentation

Add example of MHC for KCP remediation. Remove text indicating MHC is not supported for control plane nodes and explicitly call out support for remediation of KCP nodes. Add wording regarding the option for provider-supplied remediation solutions. Update the limitation and caveats section with a link to the KCP remediation guidelines for more detail. Fix minor typos and formatting issues. Ensure consistent capitalization for all headings.

Signed-off-by: Scott Lowe <[email protected]>
Co-authored-by: Cecile Robert-Michon <[email protected]>
Co-authored-by: Fabrizio Pandini <[email protected]>

docs/book/src/tasks/healthcheck.md

@@ -16,15 +16,16 @@ at the bottom of this page for full details of MachineHealthCheck limitations.
 
 ## What is a MachineHealthCheck?
 
-A MachineHealthCheck is a resource within the Cluster API which allows users to define conditions under which Machines within a Cluster should be considered unhealthy.
+A MachineHealthCheck is a resource within the Cluster API which allows users to define conditions under which Machines within a Cluster should be considered unhealthy. 
+A MachineHealthCheck is defined on a management cluster and scoped to a particular workload cluster.
 
-When defining a MachineHealthCheck, users specify a timeout for each of the conditions that they define to check on the Machine's Node,
+When defining a MachineHealthCheck, users specify a timeout for each of the conditions that they define to check on the Machine's Node;
 if any of these conditions is met for the duration of the timeout, the Machine will be remediated.
-The action of remediating a Machine should trigger a new Machine to be created, to replace the failed one.
+By default, the action of remediating a Machine should trigger a new Machine to be created to replace the failed one, but providers are allowed to plug in more sophisticated external remediation solutions. 
 
 ## Creating a MachineHealthCheck
 
-Use the following example as a basis for creating a MachineHealthCheck:
+Use the following example as a basis for creating a MachineHealthCheck for worker nodes:
 
 ```yaml
 apiVersion: cluster.x-k8s.io/v1alpha3
@@ -53,16 +54,39 @@ spec:
     timeout: 300s
 ```
 
+Use this example as the basis for defining a MachineHealthCheck for control plane nodes managed via 
+the KubeadmControlPlane:
+
+```yaml
+apiVersion: cluster.x-k8s.io/v1alpha3
+kind: MachineHealthCheck
+metadata:
+  name: capi-quickstart-kcp-unhealthy-5m
+spec:
+  clusterName: capi-quickstart
+  maxUnhealthy: 100%
+  selector:
+    matchLabels:
+      cluster.x-k8s.io/control-plane: ""
+  unhealthyConditions:
+    - type: Ready
+      status: Unknown
+      timeout: 300s
+    - type: Ready
+      status: "False"
+      timeout: 300s
+```
+
 <aside class="note warning">
 
 <h1> Important </h1>
 
 If you are defining more than one `MachineHealthCheck` for the same Cluster, make sure that the selectors **do not overlap**
-n order to prevent conflicts or unexpected behaviors when trying to remediate the same set of machines.
+in order to prevent conflicts or unexpected behaviors when trying to remediate the same set of machines.
 
 </aside>
 
-## Remediation short-circuiting
+## Remediation Short-Circuiting
 
 To ensure that MachineHealthChecks only remediate Machines when the cluster is healthy,
 short-circuiting is implemented to prevent further remediation via the `maxUnhealthy` field within the MachineHealthCheck spec.
@@ -80,15 +104,15 @@ This means the short circuiting mechanism is **disabled by default** and Machine
 
 </aside>
 
-#### With an absolute value
+#### With an Absolute Value
 
 If `maxUnhealthy` is set to `2`:
 - If 2 or fewer nodes are unhealthy, remediation will be performed
 - If 3 or more nodes are unhealthy, remediation will not be performed
 
 These values are independent of how many Machines are being checked by the MachineHealthCheck.
 
-#### With percentages
+#### With Percentages
 
 If `maxUnhealthy` is set to `40%` and there are 25 Machines being checked:
 - If 10 or fewer nodes are unhealthy, remediation will be performed
@@ -107,7 +131,7 @@ There are scenarios where remediation for a machine may be undesirable (eg. duri
 Implicit skipping when the resource is paused (using `cluster.x-k8s.io/paused` annotation):
 - When a cluster is paused, none of the machines in that cluster are considered for remediation.
 - When a machine is paused, only that machine is not considered for remediation.
-- A cluster or a machine is usually paused automatically by cluster api when it detects a migration.
+- A cluster or a machine is usually paused automatically by Cluster API when it detects a migration.
 
 Explicit skipping using `cluster.x-k8s.io/skip-remediation` annotation:
 - Users can also skip any machine for remediation by setting the `cluster.x-k8s.io/skip-remediation` for that machine.
@@ -116,10 +140,10 @@ Explicit skipping using `cluster.x-k8s.io/skip-remediation` annotation:
 
 Before deploying a MachineHealthCheck, please familiarise yourself with the following limitations and caveats:
 
-- Only Machines owned by a MachineSet will be remediated by a MachineHealthCheck
-- Control Plane Machines are currently not supported and will **not** be remediated if they are unhealthy
+- Only Machines owned by a MachineSet or a KubeadmControlPlane can be remediated by a MachineHealthCheck (since a MachineDeployment uses a MachineSet, then this includes Machines that are part of a MachineDeployment)
+- Machines managed by a KubeadmControlPlane are remediated according to [the delete-and-recreate guidelines described in the KubeadmControlPlane proposal](https://github.com/kubernetes-sigs/cluster-api/blob/master/docs/proposals/20191017-kubeadm-based-control-plane.md#remediation-using-delete-and-recreate)
 - If the Node for a Machine is removed from the cluster, a MachineHealthCheck will consider this Machine unhealthy and remediate it immediately
-- If no Node joins the cluster for a Node after the `NodeStartupTimeout`, the Machine will be remediated
+- If no Node joins the cluster for a Machine after the `NodeStartupTimeout`, the Machine will be remediated
 - If a Machine fails for any reason (if the FailureReason is set), the Machine will be remediated immediately
 
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