[SPARK-8184][SQL] Add additional function description for weekofyear

## What changes were proposed in this pull request?

Add additional function description for weekofyear.

## How was this patch tested?

 manual tests

![weekofyear](https://cloud.githubusercontent.com/assets/5399861/26525752/08a1c278-4394-11e7-8988-7cbf82c3a999.gif)

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18132 from wangyum/SPARK-8184.