add some docs(mostly from the disscussion) for the implementation

hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java

@@ -2406,6 +2406,10 @@ private void startActiveMasterManager(int infoPort) throws KeeperException {
         Threads.sleep(timeout);
       }
     }
+
+    // Here for the master startup process, we use TaskGroup to monitor the whole progress.
+    // The UI is similar to how Hadoop designed the startup page for the NameNode.
+    // See HBASE-21521 for more details.
     boolean ignoreClearStartupStatus =
       conf.getBoolean("hbase.master.ignore.clear.startup.status", true);
     startupTaskGroup = TaskGroup.createTaskGroup(ignoreClearStartupStatus);

hbase-server/src/main/java/org/apache/hadoop/hbase/monitoring/TaskGroup.java

@@ -24,11 +24,24 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+/**
+ * The {@link TaskGroup} can be seen as a big {@link MonitoredTask}, which contains a list of sub
+ * monitored tasks. The monitored tasks in the group are still be managed by the
+ * {@link TaskMonitor}, but whether to clear/expire the monitored tasks in a task group is optional.
+ * Since the monitored task already has journals, which mark the phases in a task, we still also
+ * need a task group to monitor a big task/process because the journals in a task is serial but the
+ * tasks in the task group can be parallel, then we have more flexible ability to monitor the
+ * process. Grouping the tasks is not strictly necessary but it is cleaner for presentation to
+ * operators. We might want to display the tasks in a group in a list view where each task can be
+ * collapsed (probably by default) or expanded.
+ */
 @InterfaceAudience.Private
 public class TaskGroup extends MonitoredTaskImpl {
   private static final Logger LOG = LoggerFactory.getLogger(TaskGroup.class);
 
   private final ConcurrentLinkedDeque<MonitoredTask> tasks = new ConcurrentLinkedDeque<>();
+
+  /** Whether to clear/expire the monitored tasks by {@link TaskMonitor} */
   private final boolean ignoreClearStatus;
 
   public TaskGroup(boolean ignoreClearStatus) {
@@ -48,6 +61,12 @@ public synchronized MonitoredTask addTask(String description) {
     return addTask(description, true);
   }
 
+  /**
+   * add a new task to the group, and before that might complete the last task in the group
+   * @param description      the description of the new task
+   * @param withCompleteLast whether to complete the last task in the group
+   * @return the added new task
+   */
   public synchronized MonitoredTask addTask(String description, boolean withCompleteLast) {
     if (withCompleteLast) {
       MonitoredTask previousTask = this.tasks.peekLast();