public DocumentType registerDocumentType(DocumentType docType) {
        register(docType);
        return docType;
    }
Registers a document type. Typically called by someone
importing the document types from the common Vespa repository.

@param docType The document type to register.
@return the previously registered type, or null if none was registered
document/src/main/java/com/yahoo/document/DocumentTypeManager.java