[SPARK-30588][DOC] Document CLUSTER BY Clause of SELECT statement in SQL Reference

### What changes were proposed in this pull request?
Document CLUSTER BY clause of SELECT statement in SQL Reference Guide.

### Why are the changes needed?
Currently Spark lacks documentation on the supported SQL constructs causing
confusion among users who sometimes have to look at the code to understand the
usage. This is aimed at addressing this issue.

### Does this PR introduce any user-facing change?
Yes.

**Before:**
There was no documentation for this.

**After.**
<img width="972" alt="Screen Shot 2020-01-20 at 2 59 05 PM" src="https://user-images.githubusercontent.com/14225158/72762704-7528de80-3b95-11ea-9d34-8fa0ab63d4c0.png">
<img width="972" alt="Screen Shot 2020-01-20 at 2 59 19 PM" src="https://user-images.githubusercontent.com/14225158/72762710-78bc6580-3b95-11ea-8279-2848d3b9e619.png">

### How was this patch tested?
Tested using jykyll build --serve

Closes apache#27297 from dilipbiswal/sql-ref-select-clusterby.

Authored-by: Dilip Biswal <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: dilipbiswal

docs/sql-ref-syntax-qry-select-clusterby.md

@@ -0,0 +1,88 @@
+---
+layout: global
+title: CLUSTER BY Clause
+displayTitle: CLUSTER BY Clause
+license: |
+  Licensed to the Apache Software Foundation (ASF) under one or more
+  contributor license agreements.  See the NOTICE file distributed with
+  this work for additional information regarding copyright ownership.
+  The ASF licenses this file to You under the Apache License, Version 2.0
+  (the "License"); you may not use this file except in compliance with
+  the License.  You may obtain a copy of the License at
+ 
+     http://www.apache.org/licenses/LICENSE-2.0
+ 
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+---
+The <code>CLUSTER BY</code> clause is used to first repartition the data based
+on the input expressions and then sort the data within each partition. This is
+semantically equivalent to performing a <code>DISTRIBUTE BY</code> followed by 
+a <code>SORT BY</code>. This clause only ensures that the resultant rows are
+sorted within each partition and does not guarantee a total order of output.
+
+### Syntax
+{% highlight sql %}
+CLUSTER BY { expression [ , ... ] }
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>expression</em></code></dt>
+  <dd>
+    Specifies combination of one or more values, operators and SQL functions that results in a value.
+  </dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+CREATE TABLE person (name STRING, age INT);
+INSERT INTO person VALUES 
+    ('Zen Hui', 25), 
+    ('Anil B', 18), 
+    ('Shone S', 16), 
+    ('Mike A', 25),
+    ('John A', 18), 
+    ('Jack N', 16);
+
+-- Reduce the number of shuffle partitions to 2 to illustrate the behavior of `CLUSTER BY`.
+-- It's easier to see the clustering and sorting behavior with less number of partitions.
+SET spark.sql.shuffle.partitions = 2;
+                        
+-- Select the rows with no ordering. Please note that without any sort directive, the results
+-- of the query is not deterministic. It's included here to show the difference in behavior 
+-- of a query when `CLUSTER BY` is not used vs when it's used. The query below produces rows
+-- where age column is not sorted.
+SELECT age, name FROM person;
+
+  +---+-------+
+  |age|name   |
+  +---+-------+
+  |16 |Shone S|
+  |25 |Zen Hui|
+  |16 |Jack N |
+  |25 |Mike A |
+  |18 |John A |
+  |18 |Anil B |
+  +---+-------+
+
+-- Produces rows clustered by age. Persons with same age are clustered together.
+-- In the query below, persons with age 18 and 25 are in first partition and the
+-- persons with age 16 are in the second partition. The rows are sorted based
+-- on age within each partition.
+SELECT age, name FROM person CLUSTER BY age;
+
+  +---+-------+
+  |age|name   |
+  +---+-------+
+  |18 |John A |
+  |18 |Anil B |
+  |25 |Zen Hui|
+  |25 |Mike A |
+  |16 |Shone S|
+  |16 |Jack N |
+  +---+-------+
+{% endhighlight %}