Update package doc

doc.go

@@ -9,26 +9,27 @@ The Client is used to register a task to be processed at the specified time.
 
 	client := asynq.NewClient(redis)
 
-	t := &asynq.Task{
+	t := asynq.Task{
 	    Type:    "send_email",
-	    Payload: map[string]interface{}{"recipient_id": 123},
+	    Payload: map[string]interface{}{"user_id": 42},
 	}
 
-	err := client.Process(t, time.Now().Add(10 * time.Minute))
+	err := client.Process(&t, time.Now().Add(time.Minute))
 
-The Background is used to run the background processing with a given
-handler with the specified number of workers.
+The Background is used to run the background task processing with a given
+handler.
     bg := asynq.NewBackground(redis, &asynq.Config{
-        Concurrency: 20,
+        Concurrency: 10,
     })
 
     bg.Run(handler)
 
-Handler is an interface that implements ProcessTask method that
-takes a Task and return an error. Handler should return nil if
-the processing is successful, otherwise return non-nil error
-so that the task will be retried after some delay.
+Handler is an interface with one method ProcessTask which
+takes a task and returns an error. Handler should return nil if
+the processing is successful, otherwise return a non-nil error.
+If handler returns a non-nil error, the task will be retried in the future.
 
+Example of a type that implements the Handler interface.
     type TaskHandler struct {
         // ...
     }
@@ -36,7 +37,8 @@ so that the task will be retried after some delay.
     func (h *TaskHandler) ProcessTask(task *asynq.Task) error {
         switch task.Type {
         case "send_email":
-            // send email logic
+            id, err := task.Payload.GetInt("user_id")
+            // send email
         case "generate_thumbnail":
             // generate thumbnail image
         //...