Update doc / improve bookkeeping

[brussee]

This is no longer supposed to be the case since onAttributesRetrieved was implemented:

orm/src/Entity/PersistentEntity.php

Line 162 in 82b699c

* Relies on getters and setters to update $attributes_retrieved

[brussee]

The reason is that Entities are not supposed to touch the database themselves, the Model class should handle that.

Now, AFAIK, a current shortcoming of the bookkeeping is that it only knows about what was retrieved from the database, not what was written or modified since. So for example: setting an unset attribute and then retrieving all (non-sparse) values would exclude that modified value.

Perhaps magic getters and setters can be used to improve this by updating $attributes_retrieved automagically?