YARN-9275. Add link to NodeAttributes doc in PlacementConstraints document. Contributed by Masatake Iwasaki.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/PlacementConstraints.md.vm

@@ -140,7 +140,7 @@ The semantics of these constraints are the following:
 
 $H4 Differences between node labels, node attributes and allocation tags
 
-The difference between allocation tags and node labels or node attributes (YARN-3409), is that allocation tags are attached to allocations and not to nodes. When an allocation gets allocated to a node by the scheduler, the set of tags of that allocation are automatically added to the node for the duration of the allocation. Hence, a node inherits the tags of the allocations that are currently allocated to the node. Likewise, a rack inherits the tags of its nodes. Moreover, similar to node labels and unlike node attributes, allocation tags have no value attached to them. As we show below, our constraints can refer to allocation tags, as well as node labels and node attributes.
+The difference between allocation tags and node labels or [node attributes](NodeAttributes.html), is that allocation tags are attached to allocations and not to nodes. When an allocation gets allocated to a node by the scheduler, the set of tags of that allocation are automatically added to the node for the duration of the allocation. Hence, a node inherits the tags of the allocations that are currently allocated to the node. Likewise, a rack inherits the tags of its nodes. Moreover, similar to node labels and unlike node attributes, allocation tags have no value attached to them. As we show below, our constraints can refer to allocation tags, as well as node labels and node attributes.
 
 $H3 Placement constraints API