DynamoDb enhanced client: support UpdateExpressions in single-request update

Author: anasatirbasa

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/update/UpdateExpressionResolver.java

@@ -76,6 +76,8 @@ public static Builder builder() {
      * <p><b>Backward compatibility:</b> This enhancement is not a breaking change. Without request
      * expressions, behavior is identical to previous versions.
      *
+     * <p><b>Exception scenarios:</b> DynamoDbException may be thrown when the same attribute is updated by multiple sources.
+     *
      * @return merged UpdateExpression, or empty if no updates needed
      */
     public UpdateExpression resolve() {