Clarify the appointment of the intermediate collection

Motivation:

An intermediate list is creating in the `EpollEventLoop#closeAll` to prevent ConcurrentModificationException. But this is not the obvious purpose has no comment.

Modifications:

Add comment to clarify the appointment of the intermediate collection.

Result:

More clear code.

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java

@@ -350,6 +350,8 @@ private void closeAll() {
         } catch (IOException ignore) {
             // ignore on close
         }
+        // Using the intermediate collection to prevent ConcurrentModificationException.
+        // In the `close()` method, the channel is deleted from `channels` map.
         Collection<AbstractEpollChannel> array = new ArrayList<AbstractEpollChannel>(channels.size());
 
         for (AbstractEpollChannel channel: channels.values()) {