[SPARK-33877][SQL] SQL reference documents for INSERT w/ a column list

We support a column list of INSERT for Spark v3.1.0 (See: SPARK-32976 (apache#29893)). So, this PR targets at documenting it in the SQL documents.

### What changes were proposed in this pull request?

improve doc
### Why are the changes needed?

### Does this PR introduce _any_ user-facing change?

doc
### How was this patch tested?

passing GA doc gen.

![image](https://user-images.githubusercontent.com/8326978/102954876-8994fa00-450f-11eb-81f9-931af6d1f69b.png)
![image](https://user-images.githubusercontent.com/8326978/102954900-99acd980-450f-11eb-9733-115ad37d2319.png)

![image](https://user-images.githubusercontent.com/8326978/102954935-af220380-450f-11eb-9aaa-fdae0725d41e.png)
![image](https://user-images.githubusercontent.com/8326978/102954949-bc3ef280-450f-11eb-8a0d-d7b688efa7bb.png)

Closes apache#30888 from yaooqinn/SPARK-33877.

Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

docs/sql-ref-syntax-dml-insert-into.md

@@ -26,7 +26,7 @@ The `INSERT INTO` statement inserts new rows into a table. The inserted rows can
 ### Syntax
 
 ```sql
-INSERT INTO [ TABLE ] table_identifier [ partition_spec ]
+INSERT INTO [ TABLE ] table_identifier [ partition_spec ] [ ( column_list ) ]
     { VALUES ( { value | NULL } [ , ... ] ) [ , ( ... ) ] | query }
 ```
 
@@ -40,11 +40,20 @@ INSERT INTO [ TABLE ] table_identifier [ partition_spec ]
 
 * **partition_spec**
 
-    An optional parameter that specifies a comma separated list of key and value pairs
+    An optional parameter that specifies a comma-separated list of key and value pairs
     for partitions.
 
     **Syntax:** `PARTITION ( partition_col_name  = partition_col_val [ , ... ] )`
 
+* **column_list**
+
+    An optional parameter that specifies a comma-separated list of columns belonging to the `table_identifier` table.
+
+    **Note:**The current behaviour has some limitations:
+    - All specified columns should exist in the table and not be duplicated from each other. It includes all columns except the static partition columns.
+    - The size of the column list should be exactly the size of the data from `VALUES` clause or query.
+    - The order of the column list is alterable and determines how the data from `VALUES` clause or query to be inserted by position.
+
 * **VALUES ( { value `|` NULL } [ , ... ] ) [ , ( ... ) ]**
 
     Specifies the values to be inserted. Either an explicitly specified value or a NULL can be inserted.
@@ -198,6 +207,34 @@ SELECT * FROM students;
 +-------------+--------------------------+----------+
 ```
 
+#### Insert with a column list
+
+```sql
+INSERT INTO students (address, name, student_id) VALUES
+    ('Hangzhou, China', 'Kent Yao', 11215016);
+
+SELECT * FROM students WHERE name = 'Kent Yao';
++---------+----------------------+----------+
+|     name|               address|student_id|
++---------+----------------------+----------+
+|Kent Yao |       Hangzhou, China|  11215016|
++---------+----------------------+----------+
+```
+
+#### Insert with both a partition spec and a column list
+
+```sql
+INSERT INTO students PARTITION (student_id = 11215017) (address, name) VALUES
+    ('Hangzhou, China', 'Kent Yao Jr.');
+
+SELECT * FROM students WHERE student_id = 11215017;
++------------+----------------------+----------+
+|        name|               address|student_id|
++------------+----------------------+----------+
+|Kent Yao Jr.|       Hangzhou, China|  11215017|
++------------+----------------------+----------+
+```
+
 ### Related Statements
 
 * [INSERT OVERWRITE statement](sql-ref-syntax-dml-insert-overwrite-table.html)

docs/sql-ref-syntax-dml-insert-overwrite-table.md

@@ -26,7 +26,7 @@ The `INSERT OVERWRITE` statement overwrites the existing data in the table using
 ### Syntax
 
 ```sql
-INSERT OVERWRITE [ TABLE ] table_identifier [ partition_spec [ IF NOT EXISTS ] ]
+INSERT OVERWRITE [ TABLE ] table_identifier [ partition_spec [ IF NOT EXISTS ] ] [ ( column_list ) ]
     { VALUES ( { value | NULL } [ , ... ] ) [ , ( ... ) ] | query }
 ```
 
@@ -40,11 +40,22 @@ INSERT OVERWRITE [ TABLE ] table_identifier [ partition_spec [ IF NOT EXISTS ] ]
 
 * **partition_spec**
 
-    An optional parameter that specifies a comma separated list of key and value pairs
+    An optional parameter that specifies a comma-separated list of key and value pairs
     for partitions.
 
     **Syntax:** `PARTITION ( partition_col_name [ = partition_col_val ] [ , ... ] )`
 
+* **column_list**
+
+    An optional parameter that specifies a comma-separated list of columns belonging to the `table_identifier` table.
+
+    **Note**
+
+    The current behaviour has some limitations:
+    - All specified columns should exist in the table and not be duplicated from each other. It includes all columns except the static partition columns.
+    - The size of the column list should be exactly the size of the data from `VALUES` clause or query.
+    - The order of the column list is alterable and determines how the data from `VALUES` clause or query to be inserted by position.
+
 * **VALUES ( { value `|` NULL } [ , ... ] ) [ , ( ... ) ]**
 
     Specifies the values to be inserted. Either an explicitly specified value or a NULL can be inserted.
@@ -169,6 +180,34 @@ SELECT * FROM students;
 +-----------+-------------------------+----------+
 ```
 
+#### Insert with a column list
+
+```sql
+INSERT OVERWRITE students (address, name, student_id) VALUES
+    ('Hangzhou, China', 'Kent Yao', 11215016);
+
+SELECT * FROM students WHERE name = 'Kent Yao';
++---------+----------------------+----------+
+|     name|               address|student_id|
++---------+----------------------+----------+
+|Kent Yao |       Hangzhou, China|  11215016|
++---------+----------------------+----------+
+```
+
+#### Insert with both a partition spec and a column list
+
+```sql
+INSERT OVERWRITE students PARTITION (student_id = 11215016) (address, name) VALUES
+    ('Hangzhou, China', 'Kent Yao Jr.');
+
+SELECT * FROM students WHERE student_id = 11215016;
++------------+----------------------+----------+
+|        name|               address|student_id|
++------------+----------------------+----------+
+|Kent Yao Jr.|       Hangzhou, China|  11215016|
++------------+----------------------+----------+
+```
+
 ### Related Statements
 
 * [INSERT INTO statement](sql-ref-syntax-dml-insert-into.html)