[SPARK-12148][SPARKR] fix doc after renaming DataFrame to SparkDataFrame

## What changes were proposed in this pull request?

Fixed inadvertent roxygen2 doc changes, added class name change to programming guide
Follow up of #12621

## How was this patch tested?

manually checked

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #12647 from felixcheung/rdataframe.