[MINOR][DOC] Use SparkR `TRUE` value and add default values for `StructField` in SQL Guide.

## What changes were proposed in this pull request?

In `SQL Programming Guide`, this PR uses `TRUE` instead of `True` in SparkR and adds default values of `nullable` for `StructField` in Scala/Python/R (i.e., "Note: The default value of nullable is true."). In Java API, `nullable` is not optional.

**BEFORE**
* SPARK 2.1.0 RC1
http://people.apache.org/~pwendell/spark-releases/spark-2.1.0-rc1-docs/sql-programming-guide.html#data-types

**AFTER**

* R
<img width="916" alt="screen shot 2016-12-04 at 11 58 19 pm" src="https://cloud.githubusercontent.com/assets/9700541/20877443/abba19a6-ba7d-11e6-8984-afbe00333fb0.png">

* Scala
<img width="914" alt="screen shot 2016-12-04 at 11 57 37 pm" src="https://cloud.githubusercontent.com/assets/9700541/20877433/99ce734a-ba7d-11e6-8bb5-e8619041b09b.png">

* Python
<img width="914" alt="screen shot 2016-12-04 at 11 58 04 pm" src="https://cloud.githubusercontent.com/assets/9700541/20877440/a5c89338-ba7d-11e6-8f92-6c0ae9388d7e.png

">

## How was this patch tested?

Manual.

```
cd docs
SKIP_API=1 jekyll build
open _site/index.html
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #16141 from dongjoon-hyun/SPARK-SQL-GUIDE.

(cherry picked from commit 410b7898)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>