Added docs for Setty.Mix

Author: Jan-Olof Hendig

doc/Type/Setty.pod6

@@ -165,6 +165,17 @@ Returns C<True> if the invocant contains at least one element.
     my $s2 = $s1 ∩ Set.new(4, 5);                     # set intersection operator
     say $s2.Bool;                                     # False
 
+=head2 method Mix
+
+Defined as:
+
+    method Mix(Setty:D:) returns Mix:D
+
+Returns a L<Mix|/type/Mix> containing the elements of the invocant.
+
+    my Mix $b = Set.new(1, 2, 3).Mix;
+    say $b;                                           # mix(3, 1, 2)
+
 =head1 See Also
 
 L<Sets, Bags, and Mixes|/language/setbagmix>