add example of using deleteAll

Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes

change usual code style to php code style

fix doc code according to yii2 core code style (When instantiating class it should be new MyClass(); instead of new MyClass;.)

typo

Fixes #2016: removed ability to declare scopes in ActiveRecord leaving it to ActiveQuery. Changed documentation accordingly.