Docs: Enhance documentation on identifier fields (#9478)

docs/spark-ddl.md

@@ -36,7 +36,7 @@ Spark 3 can create tables in any Iceberg catalog with the clause `USING iceberg`
 
 ```sql
 CREATE TABLE prod.db.sample (
-    id bigint COMMENT 'unique id',
+    id bigint NOT NULL COMMENT 'unique id',
     data string)
 USING iceberg;
 ```
@@ -459,7 +459,8 @@ ALTER TABLE prod.db.sample WRITE DISTRIBUTED BY PARTITION LOCALLY ORDERED BY cat
 
 ### `ALTER TABLE ... SET IDENTIFIER FIELDS`
 
-Iceberg supports setting identifier fields to a spec using `SET IDENTIFIER FIELDS`:
+Iceberg supports setting [identifier fields](https://iceberg.apache.org/spec/#identifier-field-ids) to a spec using `SET IDENTIFIER FIELDS`:
+Spark table can support Flink SQL upsert operation if the table has identifier fields.
 
 ```sql
 ALTER TABLE prod.db.sample SET IDENTIFIER FIELDS id
@@ -468,7 +469,8 @@ ALTER TABLE prod.db.sample SET IDENTIFIER FIELDS id, data
 -- multiple columns
 ```
 
-identifier fields must be `NOT NULL`, The later `ALTER` statement will overwrite the previous setting.
+Identifier fields must be `NOT NULL` columns when they are created or added. 
+The later `ALTER` statement will overwrite the previous setting.
 
 ### `ALTER TABLE ... DROP IDENTIFIER FIELDS`