Add to the PSP warning about PVs that hostPath PVs cannot be made read-only. (#19504)

Author: cjcullen

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

@@ -421,8 +421,10 @@ The **recommended minimum set** of allowed volumes for new PSPs are:
 - projected
 
 {{< warning >}}
-PodSecurityPolicy does not limit the types of `PersistentVolume` objects that may be referenced by a `PersistentVolumeClaim`.
-Only trusted users should be granted permission to create `PersistentVolume` objects.
+PodSecurityPolicy does not limit the types of `PersistentVolume` objects that
+may be referenced by a `PersistentVolumeClaim`, and hostPath type
+`PersistentVolumes` do not support read-only access mode. Only trusted users
+should be granted permission to create `PersistentVolume` objects.
 {{< /warning >}}
 
 **FSGroup** - Controls the supplemental group applied to some volumes.