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 previously used `Model` as base class and found there was quite some redundant code. I don't think we should turn to that direction again.

It's perfectly fine for `PostSearch` to extend from `Post` because it is all about searching for post data. If the search model is more generic, then sure it should extend from `Model`.