[SPARK-28802][DOC][SQL] Document DESCRIBE DATABASE statement in SQL R…

…eference

### What changes were proposed in this pull request?
Document DESCRIBE DATABASE statement in SQL Reference

### Why are the changes needed?

To complete the SQL Reference

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

#### Before
There is no documentation for this command in sql reference

#### After
![Screen Shot 2019-09-05 at 12 59 32 PM](https://user-images.githubusercontent.com/7550280/64379235-53aec800-cfe3-11e9-8a51-ea55f0455c47.png)
![Screen Shot 2019-09-05 at 12 59 45 PM](https://user-images.githubusercontent.com/7550280/64379247-58737c00-cfe3-11e9-9a51-f12c5c5bc26a.png)

### How was this patch tested?
Used jekyll build and serve to verify

Closes #25528 from kevinyu98/sql-ref-describe.

Lead-authored-by: Kevin Yu <qyu@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-database.md

@@ -18,5 +18,77 @@ license: |
   See the License for the specific language governing permissions and
   limitations under the License.
 ---
+### Description
+​
+`DESCRIBE DATABASE` statement returns the metadata of an existing database. The metadata information includes database
+name, database comment, and database location on the filesystem. If the optional `EXTENDED` option is specified, it
+returns the basic metadata information along with the database properties. The `DATABASE` and `SCHEMA` are
+interchangeable.
 
-**This page is under construction**
+### Syntax
+{% highlight sql %}
+{DESC | DESCRIBE} DATABASE [EXTENDED] db_name
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>db_name</em></code></dt>
+  <dd>
+    Specifies a name of an existing database or an existing schema in the syetem. If the name does not exist, an
+    exception is thrown.
+  </dd>
+</dl>
+
+### Example
+{% highlight sql %}
+-- Create employees DATABASE
+CREATE DATABASE employees COMMENT 'For software companies';
+
+-- Describe employees DATABASE.
+-- Returns Database Name, Description and Root location of the filesystem
+-- for the employees DATABASE.
+DESCRIBE DATABASE employees;
+  +-------------------------+-----------------------------+
+  |database_description_item|database_description_value   |
+  +-------------------------+-----------------------------+
+  |Database Name            |employees                    |
+  |Description              |For software companies       |
+  |Location                 |file:/Users/Temp/employees.db|
+  +-------------------------+-----------------------------+
+
+-- Create employees DATABASE
+CREATE DATABASE employees COMMENT 'For software companies';
+
+-- Alter employees database to set DBPROPERTIES
+ALTER DATABASE employees SET DBPROPERTIES ('Create-by' = 'Kevin', 'Create-date' = '09/01/2019');
+
+-- Describe employees DATABASE with EXTENDED option to return additional database properties
+DESCRIBE DATABASE EXTENDED employees;
+  +-------------------------+---------------------------------------------+
+  |database_description_item|database_description_value                   |
+  +-------------------------+---------------------------------------------+
+  |Database Name            |employees                                    |
+  |Description              |For software companies                       |
+  |Location                 |file:/Users/Temp/employees.db                |
+  |Properties               |((Create-by,kevin), (Create-date,09/01/2019))|
+  +-------------------------+---------------------------------------------+
+
+-- Create deployment SCHEMA
+CREATE SCHEMA deployment COMMENT 'Deployment environment';
+
+-- Describe deployment, the DATABASE and SCHEMA are interchangeable, their meaning are the same.
+DESC DATABASE deployment;
+  +-------------------------+------------------------------+
+  |database_description_item|database_description_value    |
+  +-------------------------+------------------------------+
+  |Database Name            |deployment                    |
+  |Description              |Deployment environment        |
+  |Location                 |file:/Users/Temp/deployment.db|
+  +-------------------------+------------------------------+
+
+{% endhighlight %}
+
+### Related Statements
+- [DESCRIBE FUNCTION](sql-ref-syntax-aux-describe-function.html)
+- [DESCRIBE TABLE](sql-ref-syntax-aux-describe-table.html)
+- [DESCRIBE QUERY](sql-ref-syntax-aux-describe-query.html)