tell that undefined values raise control exceptions in interpolations

doc/Language/quoting.pod6

@@ -147,6 +147,16 @@ You can also use unicode names with C<\c[]>.
     dd $s;
     OUTPUT«Str $s = "Str $s = "Camelia 💔 my ❤!"␤»
 
+Interpolation of undefined values will raise a control exception that can be
+cought in the current block with
+L<CONTROL|https://docs.perl6.org/syntax/CONTROL>.
+
+    sub niler {Nil};
+    my Str $a = niler;
+    say("$a.html", "sometext");
+    say "alive"; # this line is dead code
+    CONTROL { .die };
+
 =head2 Word quoting: qw
 X<|qw word quote>X«|< > word quote»