Add informational messages to clarify what can be filtered on.

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

Index: appengine/swarming/ui/res/imp/tasklist/task-filters.html
diff --git a/appengine/swarming/ui/res/imp/tasklist/task-filters.html b/appengine/swarming/ui/res/imp/tasklist/task-filters.html
index 2c880aeb3b828f035d51e75ab948fa90d5451727..e49cf7205b3b8a5f147451b04fed77e6b22e2eed 100644
--- a/appengine/swarming/ui/res/imp/tasklist/task-filters.html
+++ b/appengine/swarming/ui/res/imp/tasklist/task-filters.html
@@ -39,13 +39,12 @@
 <dom-module id="task-filters">
   <template>
     <style is="custom-style" include="iron-flex iron-flex-alignment iron-positioning query-column-filter-style">
-    .item.wide {
-      max-width: 400px;
-    }
-    .selector.wide {
-      min-width: 275px;
-    }
-
+      .item.wide {
+        max-width: 400px;
+      }
+      .selector.wide {
+        min-width: 275px;
+      }
     </style>
 
     <url-param name="f"
@@ -121,6 +120,12 @@
               </iron-icon>
             </div>
           </template>
+          <template is="dom-if" if="[[_secondaryItems.length]]">
+            <div class="pointable item" title="There may be more valid options than just the ones shown.">...</div>
+          </template>
+          <template is="dom-if" if="[[_showTagMessage(_primarySelected,_secondaryItems.length)]]">
+            <div class="item">Only tags can be used for filtering. <i>"[[_primarySelected]]"</i> is informational only.</div>
+          </template>
         </div>
 
         <div class="selector side-by-side"
@@ -250,7 +255,12 @@
           }
         }
         return params;
+      },
+
+      _showTagMessage: function(primarySelected, selectedItems) {
+        return primarySelected && primarySelected !== "name" && !selectedItems;
       }
+
     });
   })();
   </script>