private static Stream<NewDocumentType.Name> getReferencedDocuments(Map<String, NewDocumentType> documentDefinitions) {
        return documentDefinitions.values().stream()
                .map(NewDocumentType::getDocumentReferences)
                .flatMap(Set::stream);
    }
Performs the following validations:
 - Verify that all global documents have required redundancy
 - Verify that all referred documents are present in services.xml
 - Verify that all referred documents are global

@author bjorncs
config-model/src/main/java/com/yahoo/vespa/model/content/GlobalDistributionValidator.java