ast-walkers: add comment explaining searchOpts

Author: eush77

lib/ast-walkers.js

@@ -5,6 +5,18 @@ var TypeIndex = require('./type-index');
 var walkers = exports;
 
 
+// All walkers accept `opts` arguments (occasionally referred to as
+// `searchOpts`) which is an object with the following fields:
+//
+//   - iterator: function(node, nodeIndex, parent, [props]))
+//         function running once for each node being walked over, in order
+//
+//   - [typeIndex]: boolean=false
+//         if true, `props` will have an integer `typeIndex` field which
+//         represents a node index among all its sibling of the same type
+//
+
+
 walkers.topScan = function (node, nodeIndex, parent, opts) {
   if (parent) {
     // We would like to avoid spinning an extra loop through the starting