DynamoDb enhanced client: support UpdateExpressions in single-request update

Author: anasatirbasa

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/operations/UpdateItemOperation.java

@@ -277,15 +277,12 @@ public TransactWriteItem generateTransactWriteItem(TableSchema<T> tableSchema, O
      *   <li><b>Request expressions</b>: Includes explicit UpdateExpression from the request (if any)</li>
      * </ol>
      * 
-     * <p><b>Conflict Detection:</b>
-     * <ul>
-     *   <li>POJO vs Request conflicts are detected client-side and throw {@code IllegalArgumentException}</li>
-     *   <li>Extension vs Request conflicts are detected server-side by DynamoDB and throw {@code DynamoDbException}</li>
-     * </ul>
+     * <p><b>Conflict Detection:</b> All conflicts are detected server-side by DynamoDB and throw {@code DynamoDbException}.
      * 
      * <p><b>Attribute Filtering:</b>
-     * Attributes referenced in extension expressions are automatically excluded from REMOVE actions
-     * to prevent conflicts, even when {@code ignoreNulls} is false.
+     * When {@code ignoreNulls} is false, null POJO attributes normally generate REMOVE actions.
+     * However, attributes referenced in extension expressions are automatically excluded from
+     * REMOVE actions to prevent DynamoDB conflicts between extension and POJO operations.
      * 
      * @param tableMetadata metadata about the table structure
      * @param transformation write modification from extensions containing UpdateExpression