[MINOR][DOC] Update deprecation notes on Python/Hadoop/Scala.

## What changes were proposed in this pull request?

We had better update the deprecation notes about Python 2.6, Hadoop (before 2.6.5) and Scala 2.10 in [2.2.0-RC4](http://people.apache.org/~pwendell/spark-releases/spark-2.2.0-rc4-docs/) documentation. Since this is a doc only update, I think we can update the doc during publishing.

**BEFORE (2.2.0-RC4)**
    ![before](https://cloud.githubusercontent.com/assets/9700541/26799758/aea0dc06-49eb-11e7-8ca3-ed8ce1cc6147.png)

**AFTER**
    ![after](https://cloud.githubusercontent.com/assets/9700541/26799761/b3fef818-49eb-11e7-83c5-334f0e4768ed.png)

## How was this patch tested?

Manual.
```
SKIP_API=1 jekyll build
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #18207 from dongjoon-hyun/minor_doc_deprecation.