The developer manual is what you link to when somebody asks how something in Joomla's code works. Which means it has to cover a lot more than the framework.
Everything to do with documenting Joomla's code belongs in here. The framework, yes. But also how core features are actually built, what a new feature does and how to use it, and step-by-step tutorials for people writing their first module or plugin.
Get started, tutorials, plugins, concepts: forms, the Web Asset Manager, namespacing, services, dependency injection, events, the API, custom fields, workflows, web services. Anything in the code someone has to understand before they can build on it — including the parts nobody wrote up because whoever implemented them had already moved on to the next thing.
We sort out what is missing and what is wrong, and then we write. Having a developer in the room while you document a developer topic turns out to be remarkably effective.