[SPARK-11360][DOC] Loss of nullability when writing parquet files

This fix is to add one line to explain the current behavior of Spark SQL when writing Parquet files. All columns are forced to be nullable for compatibility reasons.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #9314 from gatorsmile/lossNull.

docs/sql-programming-guide.md

@@ -982,7 +982,8 @@ when a table is dropped.
 
 [Parquet](http://parquet.io) is a columnar format that is supported by many other data processing systems.
 Spark SQL provides support for both reading and writing Parquet files that automatically preserves the schema
-of the original data.
+of the original data. When writing Parquet files, all columns are automatically converted to be nullable for 
+compatibility reasons.
 
 ### Loading Data Programmatically