Rwerite the "Testing for Elements" section

It assumed that you have to convert something to a Set before being able
to use a set operator on it.  This is not true: set operators will take
care of conversion when needed.  In the shown example, it was actually
detrimental to first convert to a Set, because the (elem) operator can
short-circuit whenever it finds a match, so it won't have to examine all
values in the conceptual Set.

Also, in the example "so" was used: this is unnecessary, since (elem)
returns a full-blown Bool.

Also, refer to "first" if you would like to use different ways of comparison.

doc/Language/list.pod6

@@ -152,12 +152,24 @@ Since what C<for> receives is a single argument, it will be treated as a list of
 
 =head1 Testing for Elements
 
-To test for elements convert the C<List> or C<Array> to a L<C<Set>|/type/Set>
-or use a Set L<operator|/language/setbagmix>.
+To test for elements in a C<List> or C<Array>, you can use the
+L<"is element of"|https://docs.perl6.org/language/setbagmix#infix_(elem)>
+L<C<Set>|/type/Set> operator.
 
     my @a = <foo bar buzz>;
-    say @a.Set<bar buzz>; # OUTPUT: «(True True)␤»
-    say so 'bar' ∈ @a;    # OUTPUT: «True␤»
+    say 'bar' (elem) @a;    # OUTPUT: «True␤»
+    say 'bar' ∈ @a;         # same, using unicode version of operator
+
+This is the equivalent of:
+
+    'bar' (elem) @a.Set;    # convert the array to a Set first
+
+except that, if possible, it won't actually do the conversion.
+
+It basically compares the value with each element in the array using the
+L<=:=|/routine/=:=> infix operator.  If you want to use another way to
+compare values, you probably should use
+L<first|/routine/first#(List)_routine_first>.
 
 =head2 Sequences