Add missed links of commands (#4873)

Signed-off-by: Hao Ziyu <haoziyu@qiyi.com>

services/src/main/java/io/druid/cli/CliMiddleManager.java

@@ -58,7 +58,7 @@ import java.util.List;
  */
 @Command(
     name = "middleManager",
-    description = "Runs a Middle Manager, this is a \"task\" node used as part of the remote indexing service."
+    description = "Runs a Middle Manager, this is a \"task\" node used as part of the remote indexing service, see http://druid.io/docs/latest/design/middlemanager.html for a description"
 )
 public class CliMiddleManager extends ServerRunnable
 {

services/src/main/java/io/druid/cli/CliPeon.java

@@ -106,7 +106,7 @@ import java.util.Set;
 @Command(
     name = "peon",
     description = "Runs a Peon, this is an individual forked \"task\" used as part of the indexing service. "
-                  + "This should rarely, if ever, be used directly."
+                  + "This should rarely, if ever, be used directly. See http://druid.io/docs/latest/design/peons.html for a description"
 )
 public class CliPeon extends GuiceRunnable
 {

services/src/main/java/io/druid/cli/CliRouter.java

@@ -65,7 +65,7 @@ import java.util.List;
  */
 @Command(
     name = "router",
-    description = "Experimental! Understands tiers and routes things to different brokers"
+    description = "Experimental! Understands tiers and routes things to different brokers, see http://druid.io/docs/latest/development/router.html for a description"
 )
 public class CliRouter extends ServerRunnable
 {