Add informational messages to clarify what can be filtered on.

appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   This file contains most of the logic needed to create a dynamic table. It is broken up into two
   parts, a style dom-module called "dynamic-table-style" and a behavior called
   SwarmingBehaviors.DynamicTableBehavior. This behavior ties together filtering, sorting and column
   content. It also offers a few utilities to make creating the table easier. A client of these two
   parts needs to create the templates to actually draw the <table>,<tr> and so on.  See
@@ skipping 136 matching lines @@
         var dir = 1;
         if (this._sort.direction === "desc") {
           dir = -1;
         }
         var sort = this._specialSort[this._sort.name];
         if (sort) {
           return sort.bind(this)(dir, a, b);
         }
         // Default to a natural compare of the columns.
         var aCol = this._column(this._sort.name, a);
+        if (aCol === "none"){
+          // put "none" at the bottom of the sort order
+          aCol = "ZZZ";
+        }
         var bCol = this._column(this._sort.name, b);
+        if (bCol === "none"){
+          // put "none" at the bottom of the sort order
+          bCol = "ZZZ";
+        }
 
         return dir * swarming.naturalCompare(aCol, bCol);
       },
 
       _filterAndSort: function() {
         // We intentionally sort this._items (and not a copy) to allow users to
         // "chain" sorts, that is, sort by one thing and then another, and
         // have both orderings properly impact the list.
         swarming.stableSort(this._items, this._compare.bind(this));
         var items = this._items;
@@ skipping 87 matching lines @@
             var pool = this._attribute(bot, "pool");
             return pool.join(" | ");
           },
         };
       },
 
 
     }];
   })();
 </script>