[SPARK-6994][SQL] Update docs for fetching Row fields by name

add docs for https://issues.apache.org/jira/browse/SPARK-6994

Author: vidmantas zemleris <vidmantas@vinted.com>

Closes #6030 from vidma/docs/row-with-named-fields and squashes the following commits:

241b401 [vidmantas zemleris] [SPARK-6994][SQL] Update docs for fetching Row fields by name

docs/sql-programming-guide.md

@@ -367,11 +367,18 @@ val people = sc.textFile("examples/src/main/resources/people.txt").map(_.split("
 people.registerTempTable("people")
 
 // SQL statements can be run by using the sql methods provided by sqlContext.
-val teenagers = sqlContext.sql("SELECT name FROM people WHERE age >= 13 AND age <= 19")
+val teenagers = sqlContext.sql("SELECT name, age FROM people WHERE age >= 13 AND age <= 19")
 
 // The results of SQL queries are DataFrames and support all the normal RDD operations.
-// The columns of a row in the result can be accessed by ordinal.
+// The columns of a row in the result can be accessed by field index:
 teenagers.map(t => "Name: " + t(0)).collect().foreach(println)
+
+// or by field name:
+teenagers.map(t => "Name: " + t.getAs[String]("name")).collect().foreach(println)
+
+// row.getValuesMap[T] retrieves multiple columns at once into a Map[String, T]
+teenagers.map(_.getValuesMap[Any](List("name", "age"))).collect().foreach(println)
+// Map("name" -> "Justin", "age" -> 19)
 {% endhighlight %}
 
 </div>
@@ -538,7 +545,7 @@ peopleDataFrame.registerTempTable("people")
 val results = sqlContext.sql("SELECT name FROM people")
 
 // The results of SQL queries are DataFrames and support all the normal RDD operations.
-// The columns of a row in the result can be accessed by ordinal.
+// The columns of a row in the result can be accessed by field index or by field name.
 results.map(t => "Name: " + t(0)).collect().foreach(println)
 {% endhighlight %}