User stories are not all that useful (they tell you what you could do, if you knew what you were doing, which you don't).
A step by step guide/worked example kind of thing for one common goal at a time (for maybe the most dozen sufficiently different scenarios) with screenshots, arrows, every detail of what is on screen explained (or a link to more details about a graph element/UI element) from goal creation to regular data-entry (on the site or on the mobile app) and derailments and more advances stuff (retroratcheting etc.). So that a user can read a tutorial and know fairly well, what he's doing and doesn't have to learn by trial and error.
Then just a list of a hundred more common goals, witch suggest building them like one or two of the dozen well documented.
Should look something like this:
or this: (maybe with less obnoxious arrows, though)
Also a changelog referring to Beeminder version numbers, when something changes, so that a user doesn't feel (as) weirded out, when he rereads something and it isn't what he remembered.
I know, that would be a pain to create. And each small change to the Beeminder UI would probably cause two hours of work just for updating all of them. That would however be the price of good documentation. And good documentation is essential for retention.