Update gitbook

Signed-off-by: Vince Prignano <vincepri@vmware.com>

docs/book/common_code/machine_controller.md

@@ -10,16 +10,21 @@ the host with a new one matching the updated spec. If a `Machine` object is
 deleted, the corresponding `Node` should have its external resources released by
 the provider-specific controller, and should be deleted as well.
 
+Machines can be associated with a Cluster using a custom label
+`cluster.k8s.io/clusterName`. Providers using the `Cluster` controller must
+provide the label which references the name of a cluster living in the same
+namespace.
+
 {% method %}
 ## Machine
 
 `Machine` has 4 fields:
 
-`Spec` contains the desired cluster state specified by the object. While much
+`Spec` contains the desired machine state specified by the object. While much
 of the `Spec` is defined by users, unspecified parts may be filled in with
 defaults or by Controllers such as autoscalers.
 
-`Status` contains only observed cluster state and is only written by
+`Status` contains only observed machine state and is only written by
 controllers. `Status` is not the source of truth for any information, but
 instead aggregates and publishes observed state.
 
@@ -93,7 +98,7 @@ methods.
 `Delete()` will only be called when the `Machine` is in the process of being
 deleted.
 
-The definition of `Exist()` is determined by the provider.
+The definition of `Exists()` is determined by the provider.
 
 **TODO**: Provide more guidance on `Exists()`.
 
@@ -108,7 +113,7 @@ We need a diagram tracing the logic from resource creation through updates
 and finally deletion.
 {% endpanel %}
 
-0. Determine the `Cluster` associated with the `Machine`.
+0. Determine the `Cluster` associated with the `Machine` from its `cluster.k8s.io/clusterName` label.
 - If the `Machine` hasn't been deleted and doesn't have a finalizer, add one.
 - If the `Machine` is being deleted, and there is no finalizer, we're done
   - Check if the `Machine` is allowed to be deleted. [^1]