doc: mention the validation requirement for auto-timestamps (#2853)

Author: timyates

src/main/docs/guide/shared/dataUpdates/timestamps.adoc

@@ -5,3 +5,16 @@ You can annotate a property that is a date type of entity with ann:data.annotati
 You can also annotate a property that is a date type of entity with ann:data.annotation.DateUpdated[] which will be automatically populated whenever the entity is updated either via the `persist` method or when using one of the batch update methods of Micronaut Data.
 
 NOTE: If you update the entity with an external SQL statement or custom logic you will need to update the underlying `DateUpdated` column manually.
+
+=== JPA Hibernate and Entity Timestamps
+
+The ann:data.annotation.DateCreated[] and ann:data.annotation.DateUpdated[] annotations require a `ValidatorFactory` bean to be present in the application context when used with JPA Hibernate.
+This can be provided by ensuring one of
+
+dependency::io.micronaut.validation:micronaut-validation[]
+
+or
+
+dependency::io.micronaut.beanvalidation:micronaut-hibernate-validator[]
+
+is added to your project.