improved getter/setter section

@geshan @namshi/backend-read updated the doc as per last discussions
I think it should be

* A method SHOULD NOT start with `get` or `set` if it's not related to a class attribute or a service (e.g. `getDoctrine`, `getLogger`).
* Use the `load` or `find` prefix, if it's a collection/repository method call (e.g. 'loadSalesOrder`)