docs/devel: improve API documentation for QOM Hi, At the recent QEMU maintainers summit we lamented the fact it was hard to push forward with our modernising plans as legacy approaches still get introduced into the code. A lot of knowledge about the "modern" way of doing things is locked up in senior developers heads. Lets make a push to improve the developer documentation and ensure best practice is easy to find. Some sort of parable about distribution of fishes and knowledge seems appropriate here.