Update config.php
add Korean Language Translation
Update yii.php
Update config.php

add "ko" value for languages array
Update output-data-providers.md

defaultOrder this is often used for everyone
Update output-data-providers.md

Modified a bit, better?
Update UniqueValidator.php
Merge pull request #3504 from ychongsaytc/patch-1

Update UniqueValidator.php
Merge pull request #3503 from djfly/patch-5

Update output-data-providers.md
improved docs about flash messages
Merge pull request #3505 from dalli/master

[ci skip] Korean Language translation
guide WIP [skip ci]
Changed the default value of `Cache::keyPrefix` to be null.
Added support for using path alias with `FileDependency::fileName`
guide WIP [skip ci]
Finished caching guide [skip ci]
typos. [skip ci]