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 .
We create a separate search class because we don't want to mess up with the base data model class.
In this search class, you may enhance the base data model class with additional attributes specific for search purpose.

Yes, we may add additional note to clarify about this. It's also perfectly fine to create search model from scratch. The drawback is that you have to repeat attribute definition and lose some shared code in the base model class. Also, the current code is the way how Gii crud generator does.