Add informational messages to clarify what can be filtered on.

appengine/swarming/ui/res/imp/tasklist/task-list.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 in an HTML Import-able file that contains the definition
   of the following elements:
 
     <task-list>
 
@@ skipping 65 matching lines @@
           tasks="[[_items]]"
           busy="{{_busy1)}}"
           primary_map="{{_primary_map}}"
           primary_arr="{{_primary_arr}}">
         </task-list-data>
 
         <div class="horizontal layout">
           <task-filters
             primary_map="[[_primary_map]]"
             primary_arr="[[_primary_arr]]"
+            special_columns="[[_specialColumns]]"
+
             columns="{{_columns}}"
             query_params="{{_query_params}}"
             filter="{{_filter}}">
           </task-filters>
         </div>
 
         <table class="task-list">
           <thead on-sort_change="_sortChange">
           <!-- To allow for dynamic columns without having a lot of copy-pasted
           code, we break columns up into "special" and "plain" columns. Special
@@ skipping 186 matching lines @@
           var tryNum = this._attribute(task, "try_number", "-1")[0];
           if (tryNum === "0") {
             return "COMPLETED (DEDUPED)";
           }
           return "COMPLETED (SUCCESS)";
         }
         return state;
       },
     };
     var headerMap = {
+      "abandoned_ts": "Abandoned On",
+      "completed_ts": "Completed On",
+      "created_ts": "Created On",
+      "duration": "Duration",
+      "modified_ts": "Last Modified",
+      "started_ts": "Started Working On",
       "user": "Requesting User",
     };
 
     // Given a time attribute like "abandoned_ts", sortableTime returns a function
     // that compares the tasks based on the attribute.  This is used for sorting.
     function sortableTime(attr) {
       // sort times based on the string they come with, formatted like
       // "2016-08-16T13:12:40.606300" which sorts correctly.  Locale time
       // (used in the columns), does not.
       return function(dir, a, b) {
@@ skipping 133 matching lines @@
         return task.tagMap[col];
       },
 
       _taskClass: function(task) {
         return this.stateClass(this._column("state", task));
       }
     });
   })();
   </script>
 </dom-module>