added AbstractStringBuilder as example of sealed class in the JDK sin…

…ce Java 20

topics/sealedtypes.adoc

@@ -15,7 +15,7 @@ abstract sealed class AbstractStringBuilder implements Appendable, CharSequence
 }
 ----
 
-<1> Meaning that since Java 20 you can no longer extend `AbstractStringBuilder`, 
+<1> The only allowed extenders of this class are `StringBuilder` and `StringBuffer`, meaning that since Java 20 you can no longer extend `AbstractStringBuilder`, 
     although that would be difficult anyway, because it is a package private class inside the package `java.lang` inside the module `java.base`.
 
 In the code snippets below we give a complete example of a simple shape hierarchy.