Explicitly specify the components that need a change

For specifying mount propagation, changes to flags on apiserver and
kubelet are required. This PR fixes the problem.

For more comprehensive list of feature gate settings, please refer to
kubernetes#6364.

Closes: kubernetes#5766

docs/concepts/storage/volumes.md

@@ -953,13 +953,14 @@ redesigned or even removed in future releases.
 Mount propagation allows for sharing volumes mounted by a Container to
 other Containers in the same Pod, or even to other Pods on the same node.
 
-If the MountPropagation feature is disabled, volume mounts in pods are not propagated.
+If the "`MountPropagation`" feature is disabled, volume mounts in pods are not propagated.
 That is, Containers run with `private` mount propagation as described in the
 [Linux kernel documentation](https://www.kernel.org/doc/Documentation/filesystems/sharedsubtree.txt).
 
 To enable this feature, specify `MountPropagation=true` in the
-`--feature-gates` command line option. When enabled, the `volumeMounts` field
-of a Container has a new `mountPropagation` subfield. Its values are:
+`--feature-gates` command line option for the API server and kubelets.
+When enabled, the `volumeMounts` field of a Container has a new
+`mountPropagation` subfield. Its values are:
 
  * `HostToContainer` - This volume mount will receive all subsequent mounts
    that are mounted to this volume or any of its subdirectories. This is