[SPARK-5827][SQL] Add missing import in the example of SqlContext

If one tries an example by using copy&paste, throw an exception.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

Closes #4615 from maropu/AddMissingImportInSqlContext and squashes the following commits:

ab21b66 [Takeshi Yamamuro] Add missing import in the example of SqlContext

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -286,6 +286,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
    * Example:
    * {{{
    *  import org.apache.spark.sql._
+   *  import org.apache.spark.sql.types._
    *  val sqlContext = new org.apache.spark.sql.SQLContext(sc)
    *
    *  val schema =
@@ -377,6 +378,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
    * Example:
    * {{{
    *  import org.apache.spark.sql._
+   *  import org.apache.spark.sql.types._
    *  val sqlContext = new org.apache.spark.sql.SQLContext(sc)
    *
    *  val schema =