[SPARK-12071][DOC] Document the behaviour of NA in R

## What changes were proposed in this pull request?

Under Upgrading From SparkR 1.5.x to 1.6.x section added the information, SparkSQL converts `NA` in R to `null`.

## How was this patch tested?

Document update, no tests.

Author: Krishna Kalyan <krishnakalyan3@gmail.com>

Closes #13268 from krishnakalyan3/spark-12071-1.

docs/sparkr.md

@@ -386,6 +386,7 @@ You can inspect the search path in R with [`search()`](https://stat.ethz.ch/R-ma
 ## Upgrading From SparkR 1.5.x to 1.6.x
 
  - Before Spark 1.6.0, the default mode for writes was `append`. It was changed in Spark 1.6.0 to `error` to match the Scala API.
+ - SparkSQL converts `NA` in R to `null` and vice-versa.
 
 ## Upgrading From SparkR 1.6.x to 2.0