[SPARK-18723][DOC] Expanded programming guide information on wholeTex…

## What changes were proposed in this pull request?

Add additional information to wholeTextFiles in the Programming Guide. Also explain partitioning policy difference in relation to textFile and its impact on performance.

Also added reference to the underlying CombineFileInputFormat

## How was this patch tested?

Manual build of documentation and inspection in browser

```
cd docs
jekyll serve --watch
```

Author: Michal Senkyr <mike.senkyr@gmail.com>

Closes #16157 from michalsenkyr/wholeTextFilesExpandedDocs.