Release 2.6.0: Improve search results, FileSet, Table Dataset documentation

- Fixes the **icons used when returning a search**; makes the icons linked to the search result:

Staged at: [Search Results](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/search.html?q=FileSet&check_keywords=yes&area=default)

- Changes **FileSet description to clarify the nature of Input and Output formats** for FileSets:

Staged at: [File Sets](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/developers-manual/building-blocks/datasets/fileset.html)

- Fix for [CDAP-1037](https://issues.cask.co/browse/CDAP-1037): **"Table - Deleting all columns => deletes the 'row' - Not intuitive/documented"**

Staged at: [Datasets](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/developers-manual/building-blocks/datasets/table.html#delete) (very last sentence)

- Fix for [CDAP-1011](https://issues.cask.co/browse/CDAP-1011): **Installation guide should mention granting permissions to cdap user to write to temp directories**

Staged at [Quick Start: Preparing the Cluster](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/admin-manual/installation/quick-start.html#preparing-the-cluster) and [Installation: Writing to Temp Files](http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/admin-manual/installation/installation.html#install-tmp-files)