[SPARK-14965][SQL] Indicate an exception is thrown for a missing struct field

## What changes were proposed in this pull request?

Fix to ScalaDoc for StructType.

## How was this patch tested?

Built locally.

Author: Gregory Hart <greg.hart@thinkbiganalytics.com>

Closes #12758 from freastro/hotfix/SPARK-14965.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -206,8 +206,9 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
   }
 
   /**
-   * Extracts a [[StructField]] of the given name. If the [[StructType]] object does not
-   * have a name matching the given name, `null` will be returned.
+   * Extracts the [[StructField]] with the given name.
+   *
+   * @throws IllegalArgumentException if a field with the given name does not exist
    */
   def apply(name: String): StructField = {
     nameToField.getOrElse(name,
@@ -216,7 +217,9 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
 
   /**
    * Returns a [[StructType]] containing [[StructField]]s of the given names, preserving the
-   * original order of fields. Those names which do not have matching fields will be ignored.
+   * original order of fields.
+   *
+   * @throws IllegalArgumentException if a field cannot be found for any of the given names
    */
   def apply(names: Set[String]): StructType = {
     val nonExistFields = names -- fieldNamesSet
@@ -229,7 +232,9 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
   }
 
   /**
-   * Returns index of a given field
+   * Returns the index of a given field.
+   *
+   * @throws IllegalArgumentException if a field with the given name does not exist
    */
   def fieldIndex(name: String): Int = {
     nameToIndex.getOrElse(name,