[policy] Add proper deprecation notices in config entity presave bc layers

Is the bc policy the right place for this?
The deprecation policy already has a section for Code paths & behavior → that covers this case. Although that section does need an example. Is that sufficient? Does the hook_ENTITY_TYPE_presave() need to be mentioned specifically?

It should go in https://www.drupal.org/about/core/policies/core-change-policies/drupal-d... →

I got caught out by this myself recently, will find the issue and link it here...

Marking duplicate of 🐛 Add proper deprecation notices in config entity presave bc layers Active which is a much newer duplicate issue opened by me, but has more up-to-date info on it.