doc: improve cluster.workers documentation

doc/api/cluster.markdown

@@ -256,8 +256,8 @@ The method takes an optional callback argument which will be called when finishe
 
 * {Object}
 
-In the cluster all living worker objects are stored in this object by there
+A hash that stores the active worker objects, keyed by `id` field. Makes it
-`id` as the key. This makes it easy to loop through all living workers.
+easy to loop through all the workers.
 
     // Go through all workers
     function eachWorker(callback) {