[MINOR][DOC] Update `Partition Discovery` section to enumerate all available file sources

## What changes were proposed in this pull request?

All built-in data sources support `Partition Discovery`. We had better update the document to give the users more benefit clearly.

**AFTER**

<img width="906" alt="1" src="https://user-images.githubusercontent.com/9700541/30083628-14278908-9244-11e7-98dc-9ad45fe233a9.png">

## How was this patch tested?

```
SKIP_API=1 jekyll serve --watch
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #19139 from dongjoon-hyun/partitiondiscovery.