Add informational messages to clarify what can be filtered on.

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

Index: appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html
diff --git a/appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html b/appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html
index 62370ae9ebee58a522bae515de56657e996deba9..6189fe5446b2df13ac067499f061d9ac5a29bc14 100644
--- a/appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html
+++ b/appengine/swarming/ui/res/imp/common/dynamic-table-behavior.html
@@ -154,7 +154,15 @@
         }
         // 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);
       },