Add link to Nil type.

Author: fluca1978

doc/Type/Variable.pod6

@@ -26,7 +26,7 @@ Returns the name of the variable, including the sigil.
     multi sub trait_mod:<is>(Variable:D, :$default!)
 
 Sets the default value with which a variable is initialized, and to which it
-is reset when Nil is assigned to it. Trait arguments are evaluated at compile
+is reset when L<Nil|/type/Nil> is assigned to it. Trait arguments are evaluated at compile
 time. Closures won't do what you expect. They are stored as is and need to be
 called by hand.