tweak godoc

string.go

@@ -1,6 +1,7 @@
 // Package null provides a convenient way of handling null values.
 // Types in this package consider empty or zero input the same as null input.
 // Types in this package will encode to their zero value, even if null.
+// Use the nuller subpackage if you don't want this.
 package null
 
 import (