Add new *-0 behavior with example

Refs #2632

Author: JJ

doc/Language/subscripts.pod6

@@ -163,12 +163,15 @@ L<#Truncating slices> and the L<#:v> adverb.
 Positional indices are counted from the start of the collection, but there's
 also a notation for addressing elements by their position relative to the end:
 C<*-1> refers to the last element, C<*-2> to the second-to-last element, and so
-on.
+on. From version 6.d, C<*-0> refers to the element just beyond the last one
 
     my @alphabet = 'A' .. 'Z';
     say @alphabet[*-1];  # OUTPUT: «Z␤»
     say @alphabet[*-2];  # OUTPUT: «Y␤»
     say @alphabet[*-3];  # OUTPUT: «X␤»
+    @alphabet[*-0] = 'þ';
+    say @alphabet;
+    #  OUTPUT: «[A B C D E F G H I J K L M N O P Q R S T U V W X Y Z þ]␤»
 
 B<Note>: The asterisk, which is actually a L<C<Whatever>|/type/Whatever>, is
 important. Passing a bare negative integer (e.g. C<@alphabet[-1]>) like you