Add comment describing why Tree::Walker only traverse the array part.

src/Tree/Walker.nqp

@@ -3,11 +3,17 @@
 # $Id$
 
 class Tree::Walker {
+
     our multi sub walk (Tree::Walker $walker, Capture $node) {
+        # By default, just walk the children of $node.
         walkChildren($walker, $node);
     }
 
     our multi sub walkChildren (Tree::Walker $walker, Capture $tree) {
+        # We walk only the array part of the Capture by default, because
+        # the attributes of PAST::Nodes, for example, can sometimes contain
+        # cycles. In addition, in general, attributes are not used
+        # for simple sub-trees.
         my $index := 0;
         my $len := pir::elements__iP($tree);
         while ($index < $len) {