Fix task-page docs and clarify output returning nothing

appengine/swarming/ui/res/imp/taskpage/task-page-data.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-page-data>
 
@@ skipping 183 matching lines @@
         }
         // Make the duration human readable
         if (result.duration){
           result.human_duration = this._humanDuration(result.duration);
         }
         return result;
       },
 
       _parseStdout: function(stdout) {
         if (!stdout || !stdout.output) {
-          return "[No output yet]";
+          return "";
         }
         return stdout.output;
       },
 
       reloadStdout: function() {
         this._getJsonAsync("_stdout", "/api/swarming/v1/task/" +
               this.task_id + "/stdout", "_busy3", this.auth_headers);
       },
 
     });
   })();
   </script>
 </dom-module>