YARN-11001. Add docs on removing node label mapping from a node. Contributed by Manu Zhang

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/NodeLabel.md

@@ -85,7 +85,8 @@ Notes:
 ###Add/modify node-to-labels mapping to YARN
 
 * Configuring nodes to labels mapping in **Centralized** NodeLabel setup
-    * Executing ```yarn rmadmin -replaceLabelsOnNode “node1[:port]=label1 node2=label2” [-failOnUnknownNodes]```. Added label1 to node1, label2 to node2. If user don’t specify port, it adds the label to all ```NodeManagers``` running on the node. If option ```-failOnUnknownNodes``` is set, this command will fail if specified nodes are unknown.
+    * Executing ```yarn rmadmin -replaceLabelsOnNode "node1[:port]=label1 node2=label2" [-failOnUnknownNodes]``` adds label1 to node1, label2 to node2. If user doesn’t specify port, it adds the label to all ```NodeManagers``` running on the node. If option ```-failOnUnknownNodes``` is set, this command will fail if specified nodes are unknown.
+    * To remove mapping from a node, use `-replaceLabelsOnNode` but do not specify any labels. For example, ```yarn rmadmin -replaceLabelsOnNode "node1"``` removes label1 from node1.
 
 * Configuring nodes to labels mapping in **Distributed** NodeLabel setup