Private property mapping can cause issues, suggest changing to protected

The documentation recommends using private variables in entities. This can be problematic on entities with relations when using caching drivers as the proxy objects cannot access private variables and so the caching driver can throw notices like

...apc_store(): "_id" returned as member variable from __sleep() but
does not exist in ...

Making member variables protected resolves this issue when caching is enabled.

This information would be helpful on the documentation so others can be made aware of this issue. We spent a few days trying to debug the issue before understanding exactly what was going on.