Missing information about bean validation constraint annotations

Document: https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/development-guide/

There is some missing information in 13.6. Bean Validation about the constraint annotations:

1. How an existing annotation is used to set a constraint on a value
2. Where the constraints can be applied
3. What a custom constraint looks like - the annotation + validator implementation(s)
4. What rules must the annotation follow in order to be a bean validation constraint annotation
5. Constraints composition - creating new constraints by aggregating already existing ones

There are more missing items but they can be covered by another JIRA to keep it simple.