## 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.