[SPARK-21724][SQL][DOC] Adds since information in the documentation of date functions

## What changes were proposed in this pull request?

This PR adds `since` annotation in documentation so that this can be rendered as below:

<img width="290" alt="2017-08-14 6 54 26" src="https://user-images.githubusercontent.com/6477701/29267050-034c1f64-8122-11e7-862b-7dfc38e292bf.png">

## How was this patch tested?

Manually checked the documentation by `cd sql && ./create-docs.sh`.
Also, Jenkins tests are required.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18939 from HyukjinKwon/add-sinces-date-functions.