This documentation is far from complete:
are there any near future plans to update it?
Nothing concrete. We are happy to accept patches, but as far as I know nobody is currently assigned to improve the documentation.
We do not guarantee any compatibility (neither binary nor source!) for the interfaces of plugins. Many areas are basically constantly improved, so I am not even sure it makes sense to document those in a separate guide like the one you referenced (this does not include API documentation, which we also have too little of!). After all the only thing worse than missing documentation is wrong documentation.
If you have concrete questions on interfaces or how to do things, feel free to ask on "our mailing list":http://lists.qt-project.org/mailman/listinfo/qt-creator or on IRC (#qt-creator on the freenode network).