[SPARK-19843][SQL][FOLLOWUP] Classdoc for `IntWrapper` and `LongWrapper`

## What changes were proposed in this pull request?

This is as per suggestion by rxin at : https://github.com/apache/spark/pull/17184#discussion_r104841735

## How was this patch tested?

NA as this is a documentation change

Author: Tejas Patil <tejasp@fb.com>

Closes #17205 from tejasapatil/SPARK-19843_followup.