[SPARK-13748][PYSPARK][DOC] Add the description for explictly setting None for...

[SPARK-13748][PYSPARK][DOC] Add the description for explictly setting None for a named argument for a Row

## What changes were proposed in this pull request?

It seems allowed to not set a key and value for a dict to represent the value is `None` or missing as below:

``` python
spark.createDataFrame([{"x": 1}, {"y": 2}]).show()
```

```
+----+----+
|   x|   y|
+----+----+
|   1|null|
|null|   2|
+----+----+
```

However,  it seems it is not for `Row` as below:

``` python
spark.createDataFrame([Row(x=1), Row(y=2)]).show()
```

``` scala
16/06/19 16:25:56 ERROR Executor: Exception in task 6.0 in stage 66.0 (TID 316)
java.lang.IllegalStateException: Input row doesn't have expected number of values required by the schema. 2 fields are required while 1 values are provided.
    at org.apache.spark.sql.execution.python.EvaluatePython$.fromJava(EvaluatePython.scala:147)
    at org.apache.spark.sql.SparkSession$$anonfun$7.apply(SparkSession.scala:656)
    at org.apache.spark.sql.SparkSession$$anonfun$7.apply(SparkSession.scala:656)
    at scala.collection.Iterator$$anon$11.next(Iterator.scala:409)
    at scala.collection.Iterator$$anon$11.next(Iterator.scala:409)
    at org.apache.spark.sql.execution.SparkPlan$$anonfun$4.apply(SparkPlan.scala:247)
    at org.apache.spark.sql.execution.SparkPlan$$anonfun$4.apply(SparkPlan.scala:240)
    at org.apache.spark.rdd.RDD$$anonfun$mapPartitionsInternal$1$$anonfun$apply$24.apply(RDD.scala:780)
```

The behaviour seems right but it seems it might confuse users just like this JIRA was reported.

This PR adds the explanation for `Row` class.
## How was this patch tested?

N/A

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #13771 from HyukjinKwon/SPARK-13748.