Added docs for Setty.Bool

doc/Type/Setty.pod6

@@ -90,6 +90,20 @@ object, this is just the number of elements.
 Returns C<True> if C<$other> and C<self> contain all the same elements,
 and no others.
 
+=head2 method Bool
+
+Defined as:
+
+    method Bool(Setty:D:) returns Bool
+
+Returns C<True> if the invocant contains at least one element.
+
+    my $s1 = Set.new(1, 2, 3);
+    say $s1.Bool;                                     # True
+
+    my $s2 = $s1 ∩ Set.new(4, 5);                     # set intersection operator
+    say $s2.Bool;                                     # False
+
 =head1 See Also
 
 L<Sets, Bags, and Mixes|/language/setbagmix>