[doc] Add doc for adding partitions (#4374)

docs/content/flink/sql-alter.md

@@ -114,6 +114,21 @@ ALTER TABLE my_table DROP PARTITION (`id` = 1), PARTITION (`id` = 2);
 
 ```
 
+## Adding Partitions
+
+The following SQL adds the partitions of the paimon table.
+
+For flink sql, you can specify the partial columns of partition columns, and you can also specify multiple partition values at the same time, only with metastore configured metastore.partitioned-table=true.
+
+```sql
+ALTER TABLE my_table ADD PARTITION (`id` = 1);
+
+ALTER TABLE my_table ADD PARTITION (`id` = 1, `name` = 'paimon');
+
+ALTER TABLE my_table ADD PARTITION (`id` = 1), PARTITION (`id` = 2);
+
+```
+
 ## Changing Column Nullability
 
 The following SQL changes nullability of column `coupon_info`.

docs/content/spark/sql-alter.md

@@ -119,6 +119,14 @@ The following SQL drops the partitions of the paimon table. For spark sql, you n
 ALTER TABLE my_table DROP PARTITION (`id` = 1, `name` = 'paimon');
 ```
 
+## Adding Partitions
+
+The following SQL adds the partitions of the paimon table. For spark sql, you need to specify all the partition columns, only with metastore configured metastore.partitioned-table=true.
+
+```sql
+ALTER TABLE my_table ADD PARTITION (`id` = 1, `name` = 'paimon');
+```
+
 ## Changing Column Comment
 
 The following SQL changes comment of column `buy_count` to `buy count`.