Add strict inequality check for Suit enumeration

The paragraph above explicitly states that "cases are not intrinsically backed by a scalar value"; this example demonstrates that more clearly than the others.

language/enumerations.xml

@@ -105,6 +105,8 @@ $b = Suit::Spades;
 $a === $b; // true
 
 $a instanceof Suit;  // true
+
+$a !== 'Spades'; // true
 ?>
 ]]>
    </programlisting>