Update comment with exact reason for conditional logic.

Signed-off-by: Rishikesh1159 <rishireddy1159@gmail.com>

test/framework/src/main/java/org/opensearch/test/engine/MockEngineFactory.java

@@ -48,7 +48,11 @@ public MockEngineFactory(Class<? extends FilterDirectoryReader> wrapper) {
     @Override
     public Engine newReadWriteEngine(EngineConfig config) {
 
-        // When Segment Replication is enabled on an index replica shards use NRTReplicationEngine instead of InternalEngine.
+        /**
+         * Segment replication enabled replicas (i.e. read only replicas) do not use an InternalEngine so a MockInternalEngine
+         * will not work and an NRTReplicationEngine must be used instead. The primary shards for these indexes will
+         * still use a MockInternalEngine.
+         */
         return config.isReadOnlyReplica() ? new NRTReplicationEngine(config) : new MockInternalEngine(config, wrapper);
     }
 }