private static String asPrintableString(Stream<NewDocumentType.Name> documentTypes) {
        return documentTypes.map(name -> "'" + name + "'").collect(joining(", "));
    }
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