[SPARK-28804][DOCS][SQL] Document DESCRIBE QUERY in SQL Reference

### What changes were proposed in this pull request?
Document DESCRIBE QUERY 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="1234" alt="Screen Shot 2019-08-29 at 5 47 51 PM" src="https://user-images.githubusercontent.com/14225158/63985609-43e43080-ca85-11e9-8a1a-c9c15d988e24.png">
<img width="1234" alt="Screen Shot 2019-08-29 at 5 48 06 PM" src="https://user-images.githubusercontent.com/14225158/63985610-46468a80-ca85-11e9-882a-7163784f72c6.png">
<img width="1234" alt="Screen Shot 2019-08-29 at 5 48 18 PM" src="https://user-images.githubusercontent.com/14225158/63985617-49da1180-ca85-11e9-9e77-a6d6c7042a85.png">

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

Closes #25529 from dilipbiswal/ref-doc-desc-query.

Lead-authored-by: Dilip Biswal <dbiswal@us.ibm.com>
Co-authored-by: Xiao Li <gatorsmile@gmail.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

docs/sql-ref-syntax-aux-describe-query.md

@@ -19,4 +19,89 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+The `DESCRIBE QUERY` statement is used to return the metadata of output
+of a query. A shorthand `DESC` may be used instead of `DESCRIBE` to
+describe the query output.
+
+### Syntax
+{% highlight sql %}
+{DESC | DESCRIBE} [QUERY] input_statement
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>QUERY</em></code></dt>
+  <dd>This clause is optional and may be omitted.</dd>
+  <dt><code><em>input_statement</em></code></dt>
+  <dd>
+    Specifies a result set producing statement and may be one of the following: 
+    <ul>
+      <li>a <code>SELECT</code> statement</li>
+      <li>a <code>CTE(Common table expression)</code> statement</li>
+      <li>an <code>INLINE TABLE</code> statement</li>
+      <li>a <code>TABLE</code> statement</li>
+      <li>a <code>FROM</code> statement</li>
+    </ul>
+    Please refer to <a href="sql-ref-syntax-qry-select.html">select-statement</a>
+    for a detailed syntax of the query parameter.
+  </dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+-- Create table `person`
+CREATE TABLE person (name STRING , age INT COMMENT 'Age column', address STRING);
+
+-- Returns column metadata information for a simple select query
+DESCRIBE QUERY select age, sum(age) FROM person GROUP BY age;
+  +--------+---------+----------+
+  |col_name|data_type|comment   |
+  +--------+---------+----------+
+  |age     |int      |Age column|
+  |sum(age)|bigint   |null      |
+  +--------+---------+----------+
+
+-- Returns column metadata information for common table experession (`CTE`).
+DESCRIBE QUERY WITH all_names_cte
+  AS (SELECT name from person) SELECT * FROM all_names_cte;
+  +--------+---------+-------+
+  |col_name|data_type|comment|
+  +--------+---------+-------+
+  |name    |string   |null   |
+  +--------+---------+-------+
+
+-- Returns column metadata information for a inline table.
+DESC QUERY VALUES(100, 'John', 10000.20D) AS employee(id, name, salary);
+  +--------+---------+-------+
+  |col_name|data_type|comment|
+  +--------+---------+-------+
+  |id      |int      |null   |
+  |name    |string   |null   |
+  |salary  |double   |null   |
+  +--------+---------+-------+
+
+-- Returns column metadata information for `TABLE` statement.
+DESC QUERY TABLE person;
+  +--------+---------+----------+
+  |col_name|data_type|comment   |
+  +--------+---------+----------+
+  |name    |string   |null      |
+  |age     |int      |Age column|
+  |address |string   |null      |
+  +--------+---------+----------+
+
+-- Returns column metadata information for a `FROM` statement.
+-- `QUERY` clause is optional and can be omitted.
+DESCRIBE FROM person SELECT age;
+  +--------+---------+----------+
+  |col_name|data_type|comment   |
+  +--------+---------+----------+
+  |age     |int      |Age column|
+  +--------+---------+----------+
+{% endhighlight %}
+
+### Related Statements
+- [DESCRIBE DATABASE](sql-ref-syntax-aux-describe-database.html)
+- [DESCRIBE TABLE](sql-ref-syntax-aux-describe-table.html)
+- [DESCRIBE FUNCTION](sql-ref-syntax-aux-describe-function.html)