issue #3520

Adjustments code according to the standards.
Adding missing PHPDoc comment.

issue #3026

fixes #3034

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

Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

issue #2892

so information about changed values is available in `afterSave`-event.

fixes #2892

issue #2852

Fixes #2653: Fixed the bug that unsetting an unpopulated AR relation would trigger exception (qiangxue)

fixes #2623

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model

#2436 override the function `getAttributeLabel` in `BaseActiveRecord` to receive label of the attribute, which looks like `relatedMode.attribute`, from the related model