Update index.md

Author: ferdymercury

manual/io_custom_classes/index.md

@@ -217,8 +217,8 @@ The syntax is:
 ClassDef(<ClassName>, <VersionNumber>)
 {% endhighlight %}
 
-(In the case of classes inheriting from `TObject`, or from a non-TObject base class declaring itself `ClassDef*`,
-it's then better to use `ClassDefOverride` in the child instead of `ClassDef`.)
+(In the case of classes inheriting from `TObject`, or from a non-TObject base class declaring itself `ClassDef` or `ClassDefOverride` but not `ClassDefNV`,
+then it's better to use `ClassDefOverride` in the child instead of `ClassDef`. Same comment applies to `ClassDefInline` and their override variants.)
 
 The version number identifies this particular version of the class. A version number equal to 0 tells ROOT to not store the class in a root file, but only its base classes (if any).