Update active-record.md

add example of using deleteAll
Merge pull request #3249 from deerawan/master

Update active-record.md
Fixes #3230: Added `yii\filters\AccessControl::user` to support access control with different actors
Edited Introduction

Cut a sentence that seemed both redundant and vague. Could just be me.
Changed Outline

Moved “triggering” after “attaching”.
Edited "attaching event handlers"

Q: When attaching event handlers in the config, does it have to be an
anonymous function?
Edited rest of doc