I dug up the document. It contains a lot of notes, and less than is available on the mailing-list, and not polished enough to publish.
I removed the mention of it.
(There are couple of ways it can be implemented; either using a toggleable set of transactions in the stream, inserted automatically based on some rules (to implement what some people call "envelope" budgeting), or by supporting constraint assertions. It's not supported at the moment. See discussions on the mailing-list. If you can code, the first solution can be implemented without changes to Beancount - using a plugin. Also, I think Fava has budget support by using custom directives.)