[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>

docs/sql-programming-guide.md

@@ -1840,7 +1840,8 @@ You can access them by doing
   <td> The value type in Scala of the data type of this field
   (For example, Int for a StructField with the data type IntegerType) </td>
   <td>
-  StructField(<i>name</i>, <i>dataType</i>, <i>nullable</i>)
+  StructField(<i>name</i>, <i>dataType</i>, [<i>nullable</i>])<br />
+  <b>Note:</b> The default value of <i>nullable</i> is <i>true</i>.
   </td>
 </tr>
 </table>
@@ -2128,7 +2129,8 @@ from pyspark.sql.types import *
   <td> The value type in Python of the data type of this field
   (For example, Int for a StructField with the data type IntegerType) </td>
   <td>
-  StructField(<i>name</i>, <i>dataType</i>, <i>nullable</i>)
+  StructField(<i>name</i>, <i>dataType</i>, [<i>nullable</i>])<br />
+  <b>Note:</b> The default value of <i>nullable</i> is <i>True</i>.
   </td>
 </tr>
 </table>
@@ -2249,7 +2251,7 @@ from pyspark.sql.types import *
   <td> vector or list </td>
   <td>
   list(type="array", elementType=<i>elementType</i>, containsNull=[<i>containsNull</i>])<br />
-  <b>Note:</b> The default value of <i>containsNull</i> is <i>True</i>.
+  <b>Note:</b> The default value of <i>containsNull</i> is <i>TRUE</i>.
   </td>
 </tr>
 <tr>
@@ -2257,7 +2259,7 @@ from pyspark.sql.types import *
   <td> environment </td>
   <td>
   list(type="map", keyType=<i>keyType</i>, valueType=<i>valueType</i>, valueContainsNull=[<i>valueContainsNull</i>])<br />
-  <b>Note:</b> The default value of <i>valueContainsNull</i> is <i>True</i>.
+  <b>Note:</b> The default value of <i>valueContainsNull</i> is <i>TRUE</i>.
   </td>
 </tr>
 <tr>
@@ -2274,7 +2276,8 @@ from pyspark.sql.types import *
   <td> The value type in R of the data type of this field
   (For example, integer for a StructField with the data type IntegerType) </td>
   <td>
-  list(name=<i>name</i>, type=<i>dataType</i>, nullable=<i>nullable</i>)
+  list(name=<i>name</i>, type=<i>dataType</i>, nullable=[<i>nullable</i>])<br />
+  <b>Note:</b> The default value of <i>nullable</i> is <i>TRUE</i>.
   </td>
 </tr>
 </table>