private static void verifyReferredDocumentsArePresent(Map<String, NewDocumentType> documentDefinitions) {
        Set<NewDocumentType.Name> unknowDocuments = getReferencedDocuments(documentDefinitions)
                .filter(name -> !documentDefinitions.containsKey(name.toString()))
                .collect(Collectors.toCollection(() -> new LinkedHashSet<>()));
        if (!unknowDocuments.isEmpty()) {
            throw new IllegalArgumentException("The following document types are referenced from other documents, " +
                    "but are not listed in services.xml: " + asPrintableString(unknowDocuments.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