[SPARK-31753][SQL][DOCS][FOLLOW-UP] Add missing keywords in the SQL docs

### What changes were proposed in this pull request?
update sql-ref docs, the following key words will be added in this PR.

CLUSTERED BY
SORTED BY
INTO num_buckets BUCKETS

### Why are the changes needed?
let more users know the sql key words usage

### Does this PR introduce _any_ user-facing change?
No
![image](https://user-images.githubusercontent.com/46367746/94428281-0a6b8080-01c3-11eb-9ff3-899f8da602ca.png)
![image](https://user-images.githubusercontent.com/46367746/94428285-0d667100-01c3-11eb-8a54-90e7641d917b.png)
![image](https://user-images.githubusercontent.com/46367746/94428288-0f303480-01c3-11eb-9e1d-023538aa6e2d.png)

### How was this patch tested?
generate html test

Closes apache#29883 from GuoPhilipse/add-sql-missing-keywords.

Lead-authored-by: GuoPhilipse <[email protected]>
Co-authored-by: GuoPhilipse <[email protected]>
Signed-off-by: Takeshi Yamamuro <[email protected]>

docs/sql-ref-syntax-ddl-create-table-datasource.md

@@ -67,7 +67,12 @@ as any order. For example, you can write COMMENT table_comment after TBLPROPERTI
 
 * **SORTED BY**
 
-    Determines the order in which the data is stored in buckets. Default is Ascending order.
+    Specifies an ordering of bucket columns. Optionally, one can use ASC for an ascending order or DESC for a descending order after any column names in the SORTED BY clause.
+    If not specified, ASC is assumed by default.
+
+* **INTO num_buckets BUCKETS**
+
+    Specifies buckets numbers, which is used in `CLUSTERED BY` clause.
 
 * **LOCATION**
 

docs/sql-ref-syntax-ddl-create-table-hiveformat.md

@@ -31,6 +31,9 @@ CREATE [ EXTERNAL ] TABLE [ IF NOT EXISTS ] table_identifier
     [ COMMENT table_comment ]
     [ PARTITIONED BY ( col_name2[:] col_type2 [ COMMENT col_comment2 ], ... ) 
         | ( col_name1, col_name2, ... ) ]
+    [ CLUSTERED BY ( col_name1, col_name2, ...) 
+        [ SORTED BY ( col_name1 [ ASC | DESC ], col_name2 [ ASC | DESC ], ... ) ] 
+        INTO num_buckets BUCKETS ]
     [ ROW FORMAT row_format ]
     [ STORED AS file_format ]
     [ LOCATION path ]
@@ -65,6 +68,21 @@ as any order. For example, you can write COMMENT table_comment after TBLPROPERTI
 
     Partitions are created on the table, based on the columns specified.
 
+* **CLUSTERED BY**
+
+    Partitions created on the table will be bucketed into fixed buckets based on the column specified for bucketing.
+
+    **NOTE:** Bucketing is an optimization technique that uses buckets (and bucketing columns) to determine data partitioning and avoid data shuffle.
+
+* **SORTED BY**
+
+    Specifies an ordering of bucket columns. Optionally, one can use ASC for an ascending order or DESC for a descending order after any column names in the SORTED BY clause.
+    If not specified, ASC is assumed by default.
+
+* **INTO num_buckets BUCKETS**
+
+    Specifies buckets numbers, which is used in `CLUSTERED BY` clause.
+
 * **row_format**    
 
     Use the `SERDE` clause to specify a custom SerDe for one table. Otherwise, use the `DELIMITED` clause to use the native SerDe and specify the delimiter, escape character, null character and so on.
@@ -203,6 +221,20 @@ CREATE EXTERNAL TABLE family (id INT, name STRING)
     STORED AS INPUTFORMAT 'com.ly.spark.example.serde.io.SerDeExampleInputFormat'
         OUTPUTFORMAT 'com.ly.spark.example.serde.io.SerDeExampleOutputFormat'
     LOCATION '/tmp/family/';
+
+--Use `CLUSTERED BY` clause to create bucket table without `SORTED BY`
+CREATE TABLE clustered_by_test1 (ID INT, AGE STRING)
+    CLUSTERED BY (ID)
+    INTO 4 BUCKETS
+    STORED AS ORC
+
+--Use `CLUSTERED BY` clause to create bucket table with `SORTED BY`
+CREATE TABLE clustered_by_test2 (ID INT, NAME STRING)
+    PARTITIONED BY (YEAR STRING)
+    CLUSTERED BY (ID, NAME)
+    SORTED BY (ID ASC)
+    INTO 3 BUCKETS
+    STORED AS PARQUET
 ```
 
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