static TypeSignature ofUnresolved(String unresolvedTypeName) {
        requireNonNull(unresolvedTypeName, "unresolvedTypeName");
        return new DefaultTypeSignature(TypeSignatureType.UNRESOLVED, '?' + unresolvedTypeName);
    }
Creates a new unresolved type signature with the specified type name.
core/src/main/java/com/linecorp/armeria/server/docs/TypeSignature.java