Add informational messages to clarify what can be filtered on.

appengine/swarming/ui/res/imp/tasklist/task-list.html

Index: appengine/swarming/ui/res/imp/tasklist/task-list.html
diff --git a/appengine/swarming/ui/res/imp/tasklist/task-list.html b/appengine/swarming/ui/res/imp/tasklist/task-list.html
index 203a6a98f90e498fe132a794e164bb39f6696990..5b09a4d33e1d7b6f7b4f5d1b77143c2c1482c58f 100644
--- a/appengine/swarming/ui/res/imp/tasklist/task-list.html
+++ b/appengine/swarming/ui/res/imp/tasklist/task-list.html
@@ -83,6 +83,8 @@
           <task-filters
             primary_map="[[_primary_map]]"
             primary_arr="[[_primary_arr]]"
+            special_columns="[[_specialColumns]]"
+
             columns="{{_columns}}"
             query_params="{{_query_params}}"
             filter="{{_filter}}">
@@ -289,6 +291,12 @@
       },
     };
     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",
     };