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 .
@qiangxue but why it should extend from `Post` since it only hold things that dont have anything to deal with entity, also usually in Yii people like to create entities with a lot of validation rules without splitting them into separated form models, this can be a big pain when you will call `->validate()` in `search()` method. So you will need to rewrite `rules()` completly or merge them very accuratly. Almost always rules will be like `safe` or with small restrict checks on user input.
Once again yes, it is fine that view model extends from entity or it can hold it like a composite, but we dont have here view model. We have only functionality called `search model`, that dont need to have some entity features in it. 
By making extending we are introducing possible misunderstanding so in other use - cases users will be doing extending and saying `ah, i saw it in the guide, what can go wrong?`.
So all code that you think is redundand , is not, because it is driven by UI use - case: labels, validation rules and other. So i dont think that we should keep extending here.