Fix message for stalled shutdown (elastic#89254)

Today if a node shutdown is stalled due to unmoveable shards then we say
to use the allocation explain API to find details. In fact, since elastic#78727
we include the allocation explanation in the response already so we
should tell users just to look at that instead. This commit adjusts the
message to address this.

docs/changelog/89254.yaml

@@ -0,0 +1,5 @@
+pr: 89254
+summary: Fix message for stalled shutdown
+area: Infra/Node Lifecycle
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/cluster/metadata/ShutdownShardMigrationStatus.java

@@ -24,6 +24,8 @@
 public class ShutdownShardMigrationStatus implements Writeable, ToXContentObject {
     private static final Version ALLOCATION_DECISION_ADDED_VERSION = Version.V_7_16_0;
 
+    public static final String NODE_ALLOCATION_DECISION_KEY = "node_allocation_decision";
+
     private final SingleNodeShutdownMetadata.Status status;
     private final long shardsRemaining;
     @Nullable
@@ -83,7 +85,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
             builder.field("explanation", explanation);
         }
         if (Objects.nonNull(allocationDecision)) {
-            builder.startObject("node_allocation_decision");
+            builder.startObject(NODE_ALLOCATION_DECISION_KEY);
             {
                 allocationDecision.toXContent(builder, params);
             }

x-pack/plugin/shutdown/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/shutdown/NodeShutdownIT.java

@@ -367,9 +367,10 @@ public void testStalledShardMigrationProperlyDetected() throws Exception {
                 ObjectPath.eval("nodes.0.shard_migration.explanation", status),
                 allOf(
                     containsString(indexName),
-                    containsString("cannot move, use the Cluster Allocation Explain API on this shard for details")
+                    containsString("cannot move, see [node_allocation_decision] for details or use the cluster allocation explain API")
                 )
             );
+            assertThat(ObjectPath.eval("nodes.0.shard_migration.node_allocation_decision", status), notNullValue());
         }
 
         // Now update the allocation requirements to unblock shard relocation

x-pack/plugin/shutdown/src/main/java/org/elasticsearch/xpack/shutdown/TransportGetShutdownStatusAction.java

@@ -49,6 +49,7 @@
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.stream.Collectors;
 
+import static org.elasticsearch.cluster.metadata.ShutdownShardMigrationStatus.NODE_ALLOCATION_DECISION_KEY;
 import static org.elasticsearch.core.Strings.format;
 
 public class TransportGetShutdownStatusAction extends TransportMasterNodeAction<
@@ -291,10 +292,11 @@ static ShutdownShardMigrationStatus shardMigrationStatus(
                 SingleNodeShutdownMetadata.Status.STALLED,
                 totalRemainingShards,
                 format(
-                    "shard [%s] [%s] of index [%s] cannot move, use the Cluster Allocation Explain API on this shard for details",
+                    "shard [%s] [%s] of index [%s] cannot move, see [%s] for details or use the cluster allocation explain API",
                     shardRouting.shardId().getId(),
                     shardRouting.primary() ? "primary" : "replica",
-                    shardRouting.index().getName()
+                    shardRouting.index().getName(),
+                    NODE_ALLOCATION_DECISION_KEY
                 ),
                 decision
             );