fixed note about search class

Fixed some notes to avoud misunderstanding and confusions by developers . Also there was an issue about this one , but currently i cant find it .
i dont understand you,:

>The drawback is that you have to repeat attribute definition and lose some shared code in the base model class.

what kind of ? Search class is only for search, active provider returns needed active records. I would rather say that extending here is bad rather than good, because it is even not a view-model. But once again it depends on Yii suggestions and postion, if we want to position it as fw that is best only for CRUD and small projects than fine, if no - than we should not put in guide things that are not good and only acceptable in rapid development. 
Should i create separated PR to adjust guide ? Can you point in numbers what should be adjusted ?