Added javadoc and test descriptions

Signed-off-by: Shubh Sahu <shubhvs@amazon.com>

Author: Shubh Sahu

server/src/internalClusterTest/java/org/opensearch/remotemigration/ResizeIndexMigrationTestCase.java

@@ -32,6 +32,10 @@ public class ResizeIndexMigrationTestCase extends MigrationBaseTestCase {
     private final static String STRICT_MODE = "strict";
     private final static String MIXED_MODE = "mixed";
 
+    /*
+    * This test will verify the resize request failure, when cluster mode is mixed
+    * and index is on DocRep node, and migration to remote store is in progress.
+    * */
     public void testFailResizeIndexWhileDocRepToRemoteStoreMigration() throws Exception {
 
         internalCluster().setBootstrapClusterManagerNodeIndex(0);
@@ -120,6 +124,10 @@ public void testFailResizeIndexWhileDocRepToRemoteStoreMigration() throws Except
         );
     }
 
+    /*
+     * This test will verify the resize request failure, when cluster mode is mixed
+     * and index is on Remote Store node, and migration to DocRep node is in progress.
+     * */
     public void testFailResizeIndexWhileRemoteStoreToDocRepMigration() throws Exception {
 
         addRemote = true;

server/src/main/java/org/opensearch/action/admin/indices/shrink/TransportResizeAction.java

@@ -374,6 +374,18 @@ protected String getClusterManagerActionName(DiscoveryNode node) {
         return super.getClusterManagerActionName(node);
     }
 
+    /**
+     * Reject resize request if cluster mode is [Mixed] and migration direction is [RemoteStore] and index is not on
+     * REMOTE_STORE_ENABLED node or [DocRep] and index is on REMOTE_STORE_ENABLED node.
+     * @param type resize type
+     * @param sourceIndexMetadata source index's metadata
+     * @param clusterSettings cluster settings
+     * @throws IllegalStateException if cluster mode is [Mixed] and migration direction is [RemoteStore] or [DocRep] and
+     *                               index's SETTING_REMOTE_STORE_ENABLED is not equal to the migration direction's value.
+     *                               For example, if migration direction is [RemoteStore] and index's SETTING_REMOTE_STORE_ENABLED
+     *                               is false, then throw IllegalStateException. If migration direction is [DocRep] and
+     *                               index's SETTING_REMOTE_STORE_ENABLED is true, then throw IllegalStateException.
+     */
     private static void validateClusterModeSettings(
         final ResizeType type,
         IndexMetadata sourceIndexMetadata,