Standard library documentation updates.

core/builtins/native/kotlin/Nothing.kt

@@ -17,6 +17,7 @@
 package kotlin
 
 /**
- * Nothing has no instances
+ * Nothing has no instances. You can use Nothing to represent "a value that never exists": for example,
+ * if a function has the return type of Nothing, it means that it never returns (always throws an exception).
  */
 public class Nothing private () {}