simplify comment and code

the comment was mostly describing invariants for replacing group.top()
with group.all() + computeOrderedGroups + slice (9d07086)

slice's second parameter has a reasonable default
for dc-js#1184

src/cap-mixin.js

@@ -50,36 +50,17 @@ dc.capMixin = function (_chart) {
         return _chart.valueAccessor()(d, i);
     };
 
-    /**
-     * Base function orders by crossfilter's group.all.
-     * This returns all groups, in ascending order, by value.
-     *
-     * This function is expected to return items in the same direction,
-     * ascending but the N biggest where N is cap.
-     *
-     * Since computeOrderedGroups is using crossfilter's quicksort, the
-     * sorted items will always be in ascending order.
-     *
-     * This means if we cap, we need to take from N to the end of the array.
-     *
-     *  Index
-     *  0-------N-------length
-     *
-     *  Value
-     *  small -> big
-     *
-     */
+    // return N biggest groups, where N is the cap, sorted in ascending order.
     _chart.data(function (group) {
         if (_cap === Infinity) {
             return _chart._computeOrderedGroups(group.all());
         } else {
-            var topRows = group.all(); // ordered by crossfilter group order (default value)
-
-            topRows = _chart._computeOrderedGroups(topRows); // re-order using ordering (default key)
+            var topRows = group.all(); // in key order
+            topRows = _chart._computeOrderedGroups(topRows); // re-order using ordering (defaults to key)
 
             if (_cap) {
                 var start = Math.max(0, topRows.length - _cap);
-                topRows = topRows.slice(start, topRows.length);
+                topRows = topRows.slice(start);
             }
 
             if (_othersGrouper) {