[SPARK-12970][DOCS] Fix the example in SturctType APIs for Scala and Java

## What changes were proposed in this pull request?

This PR fixes both,

javadoc8 break

```
[error] .../spark/sql/hive/target/java/org/apache/spark/sql/hive/FindHiveSerdeTable.java:3: error: reference not found
[error]  * Replaces {link SimpleCatalogRelation} with {link MetastoreRelation} if its table provider is hive.
```

and the example in `StructType` as a self-contained example as below:

```scala
import org.apache.spark.sql._
import org.apache.spark.sql.types._

val struct =
  StructType(
    StructField("a", IntegerType, true) ::
    StructField("b", LongType, false) ::
    StructField("c", BooleanType, false) :: Nil)

// Extract a single StructField.
val singleField = struct("b")
// singleField: StructField = StructField(b,LongType,false)

// If this struct does not have a field called "d", it throws an exception.
struct("d")
// java.lang.IllegalArgumentException: Field "d" does not exist.
//   ...

// Extract multiple StructFields. Field names are provided in a set.
// A StructType object will be returned.
val twoFields = struct(Set("b", "c"))
// twoFields: StructType =
//   StructType(StructField(b,LongType,false), StructField(c,BooleanType,false))

// Any names without matching fields will throw an exception.
// For the case shown below, an exception is thrown due to "d".
struct(Set("b", "c", "d"))
// java.lang.IllegalArgumentException: Field "d" does not exist.
//    ...
```

```scala
import org.apache.spark.sql._
import org.apache.spark.sql.types._

val innerStruct =
  StructType(
    StructField("f1", IntegerType, true) ::
    StructField("f2", LongType, false) ::
    StructField("f3", BooleanType, false) :: Nil)

val struct = StructType(
  StructField("a", innerStruct, true) :: Nil)

// Create a Row with the schema defined by struct
val row = Row(Row(1, 2, true))
```

Also, now when the column is missing, it throws an exception rather than ignoring.

## How was this patch tested?

Manually via `sbt unidoc`.

- Scaladoc

  <img width="665" alt="2017-01-26 12 54 13" src="https://cloud.githubusercontent.com/assets/6477701/22297905/1245620e-e362-11e6-9e22-43bb8d9871af.png">

- Javadoc

  <img width="722" alt="2017-01-26 12 54 27" src="https://cloud.githubusercontent.com/assets/6477701/22297899/0fd87e0c-e362-11e6-9033-7590bda1aea6.png">

  <img width="702" alt="2017-01-26 12 54 32" src="https://cloud.githubusercontent.com/assets/6477701/22297900/0fe14154-e362-11e6-9882-768381c53163.png">

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16703 from HyukjinKwon/SPARK-12970.