Document AllowedFlexVolumes in PSP

Closes: kubernetes#6447

docs/concepts/policy/pod-security-policy.md

@@ -28,6 +28,7 @@ administrator to control the following:
 | Usage of host networking and ports                  | [`hostNetwork`, `hostPorts`](#host-namespaces) |
 | Usage of volume types                               | [`volumes`](#volumes-and-file-systems)      |
 | Usage of the host filesystem                        | [`allowedHostPaths`](#volumes-and-file-systems) |
+| Usage of FlexVolume drivers                         | [`allowedFlexVolumes`](#flexvolume-drivers) |
 | Allocating an FSGroup that owns the pod's volumes   | [`fsGroup`](#volumes-and-file-systems)      |
 | Requiring the use of a read only root file system   | [`readOnlyRootFilesystem`](#volumes-and-file-systems) |
 | The user and group IDs of the container             | [`runAsUser`, `supplementalGroups`](#users-and-groups) |
@@ -417,6 +418,20 @@ containers, and abusing the credentials of system services, such as Kubelet._
 **ReadOnlyRootFilesystem** - Requires that containers must run with a read-only
 root filesystem (i.e. no writeable layer).
 
+### FlexVolume drivers
+
+When the [`Volumes`](#volumes-and-file-systems) field contains `flexVolume` in
+its list value, the cluster admin can further specify which driver(s) is permitted
+by setting the `allowedFlexVolumes` field.
+
+**AllowedFlexVolumes** - Provides a whitelist of allowed FlexVolumes. Empty or
+nil indicates that all FlexVolume drivers may be used. For example, the following
+setting only permits the `examle/fast_cache` driver to be used on nodes:
+
+```yaml
+allowedFlexVolumes: [ "example/fast_cache" ]
+```
+
 ### Users and groups
 
 **RunAsUser** - Controls the what user ID containers run as.