[SPARK-15485][SQL][DOCS] Spark SQL Configuration

#### What changes were proposed in this pull request?
So far, the page Configuration in the official documentation does not have a section for Spark SQL.
http://spark.apache.org/docs/latest/configuration.html

For Spark users, the information and default values of these public configuration parameters are very useful. This PR is to add this missing section to the configuration.html.

rxin yhuai marmbrus

#### How was this patch tested?
Below is the generated webpage.
<img width="924" alt="screenshot 2016-05-23 11 35 57" src="https://cloud.githubusercontent.com/assets/11567269/15480492/b08fefc4-20da-11e6-9fa2-7cd5b699ed35.png">
<img width="914" alt="screenshot 2016-05-23 11 37 38" src="https://cloud.githubusercontent.com/assets/11567269/15480499/c5f9482e-20da-11e6-95ff-10821add1af4.png">
<img width="923" alt="screenshot 2016-05-23 11 36 11" src="https://cloud.githubusercontent.com/assets/11567269/15480506/cbd81644-20da-11e6-9d27-effb716b2fac.png">
<img width="920" alt="screenshot 2016-05-23 11 36 18" src="https://cloud.githubusercontent.com/assets/11567269/15480511/d013e332-20da-11e6-854a-cf8813c46f36.png">

Author: gatorsmile <gatorsmile@gmail.com>

Closes #13263 from gatorsmile/configurationSQL.

docs/configuration.md

@@ -1480,6 +1480,48 @@ Apart from these, the following properties are also available, and may be useful
 </table>
 
 
+#### Spark SQL
+Running the <code>SET -v</code> command will show the entire list of the SQL configuration.
+
+<div class="codetabs">
+<div data-lang="scala"  markdown="1">
+
+{% highlight scala %}
+// spark is an existing SparkSession
+spark.sql("SET -v").show(numRows = 200, truncate = false)
+{% endhighlight %}
+
+</div>
+
+<div data-lang="java"  markdown="1">
+
+{% highlight java %}
+// spark is an existing SparkSession
+spark.sql("SET -v").show(200, false);
+{% endhighlight %}
+</div>
+
+<div data-lang="python"  markdown="1">
+
+{% highlight python %}
+# spark is an existing SparkSession
+spark.sql("SET -v").show(n=200, truncate=False)
+{% endhighlight %}
+
+</div>
+
+<div data-lang="r"  markdown="1">
+
+{% highlight r %}
+# sqlContext is an existing sqlContext.
+properties <- sql(sqlContext, "SET -v")
+showDF(properties, numRows = 200, truncate = FALSE)
+{% endhighlight %}
+
+</div>
+</div>
+
+
 #### Spark Streaming
 <table class="table">
 <tr><th>Property Name</th><th>Default</th><th>Meaning</th></tr>