Improves explanation of reduce and checks examples

As a matter of fact, #1399 is no longer relevant or has been
solved. So it closes #1399

doc/Type/List.pod6

@@ -831,13 +831,16 @@ say <01 11 111 2 20 02>.sort( { .Int, .comb.sum, .Str } );
 
 Defined as:
 
-    multi sub    reduce(&with, *@values)
-    multi method reduce(List:D: &with)
+    multi method reduce(Any:U: & --> Nil)
+    multi method reduce(Any:D: &with)
+    multi sub reduce (&with, +list)
 
-Generates a single "combined" value from a list of arbitrarily many of values,
-by iteratively applying a function which knows how to combine I<two> values.
+The first form is obviously a no-op. The second form generates a single
+"combined" value from a list of arbitrarily many values, by iteratively
+applying a function which knows how to combine I<two> values.
 
-If C<@values> contains just a single element, the operator is applied to that single element if possible; if not, it returns the element itself.
+If C<@values> contains just a single element, the operator is applied to that
+single element if possible; if not, it returns the element itself.
 
     say [-] <10 5 3>; #OUTPUT: 2␤
     say [-] 10;       #OUTPUT: 10␤
@@ -877,8 +880,9 @@ metaoperator provides a syntactic shortcut:
     say reduce &[+], @numbers; # operator does not need explicit identity
     say [+] @numbers;          # most people write it this way
 
-Since C<reduce> is an implicit loop, it responds to C<next>, C<last> and
-C<redo> statements inside C<&with>:
+Please note also the use of C<reduce> in sub form. Since C<reduce> is an
+implicit loop, it responds to C<next>, C<last> and C<redo> statements inside
+C<&with>:
 
     say (2,3,4,5).reduce: { last if $^a > 7; $^a + $^b }; # says 9