io.write returns number of written bytes on error

manual/manual.of

@@ -8699,6 +8699,9 @@ Writes the value of each of its arguments to @id{file}.
 The arguments must be strings or numbers.
 
 In case of success, this function returns @id{file}.
+Otherwise, it returns four values:
+@fail, the error message, the error code,
+and the number of bytes it was able to write.
 
 }