Indicate Nil/Failure are exempt from return constraints

doc/Language/functions.pod6

@@ -187,6 +187,12 @@ Attempting to return values of another type will cause a compilation error.
 
     sub foo() returns Int { "a"; }; foo; # Type check fails
 
+Note that C<Nil> and C<Failure> are excempt from return type constraints and
+can be returned from any routine, regardless of its constraint:
+
+    sub foo() returns Int { fail   }; foo; # Failure returned
+    sub foo() returns Int { return }; foo; # Nil returned
+
 =head2 X<Multi-dispatch|declarator,multi>
 
 Perl 6 allows you to write several routines with the same name, but different