[SPARK-28814][SQL][DOC] Document SET/RESET in SQL Reference

### What changes were proposed in this pull request?
Document SET/REST 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:

**SET**
![image](https://user-images.githubusercontent.com/29914590/65037551-94a3c680-d96b-11e9-9d59-9f7af5185e06.png)
![image](https://user-images.githubusercontent.com/29914590/64858792-fb607180-d645-11e9-8a53-8cf87a166fc1.png)

**RESET**
![image](https://user-images.githubusercontent.com/29914590/64859019-b12bc000-d646-11e9-8cb4-73dc21830067.png)

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

Closes #25606 from sharangk/resetDoc.

Authored-by: sharangk <sharan.gk@gmail.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

docs/sql-ref-syntax-aux-conf-mgmt-reset.md

@@ -19,4 +19,20 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+Reset all the properties specific to the current session to their default values. After RESET command, executing SET command will output empty.
+
+### Syntax
+{% highlight sql %}
+RESET
+{% endhighlight %}
+
+
+### Examples
+{% highlight sql %}
+-- Reset all the properties specific to the current session to their default values.
+RESET;
+{% endhighlight %}
+
+### Related Statements
+- [SET](sql-ref-syntax-aux-conf-mgmt-set.html)

docs/sql-ref-syntax-aux-conf-mgmt-set.md

@@ -19,4 +19,51 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+The SET command sets a property, returns the value of an existing property or returns all SQLConf properties with value and meaning.
+
+### Syntax
+{% highlight sql %}
+SET
+SET [ -v ]
+SET property_key[ = property_value ]
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>-v</em></code></dt>
+  <dd>Outputs the key, value and meaning of existing SQLConf properties.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>property_key</em></code></dt>
+  <dd>Returns the value of specified property key.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>property_key=property_value</em></code></dt>
+  <dd>Sets the value for a given property key. If an old value exists for a given property key, then it gets overridden by the new value.</dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+-- Set a property.
+SET  spark.sql.variable.substitute=false;
+
+-- List all SQLConf properties with value and meaning.
+SET -v;
+
+-- List all SQLConf properties with value for current session.
+SET;
+
+-- List the value of specified property key.
+SET  spark.sql.variable.substitute;
+    +--------------------------------+--------+
+    |              key               | value  |
+    +--------------------------------+--------+
+    | spark.sql.variable.substitute  | false  |
+    +--------------------------------+--------+
+{% endhighlight %}
+
+### Related Statements
+- [RESET](sql-ref-syntax-aux-conf-mgmt-reset.html)