Document stateful service problems

[jmoore]

Several things were learned while trying to get the RenderingBean? functioning :

• #174
• #192
• #193
• #194
• #195

This needs to be codifed so the next stateful service isn't such a pain.

[jmoore]

#330 can be added to. Basically it's not clear enough that :

1. RenderingEngineImpl is wrapped (see #197 for if we can undo that)
2. Entities passing out of stateful services need to be carefully copied

[jmoore]

Still fixes to be done (and bugs to be found) in milestone:3.0-M4 Rescheduling.