With templates, it's easy, just create a /doc page and transclude it.
Now with a Module, everything under the Module: namespace appears to be code.
Is there a standard way to document how to use a module, and how to edit the data when there is? The 'most useful' place would be in the code directly, but that's not allowing for pretty documentation.
in the code. if you want some prettization, you can put the docs on the template or template/doc page. if you have a template for your module, of course. (you supposed to have at least one, just to shorten #invoke: call)
Wikipedia has added /doc support for Modules (see Module:Bananas/doc at Wikipedia), but FANDOM is still running on an old engine, so it doesn't. It is possible FANDOM will inherit that mechanism when it modernizes, but putting documentation in the primary #invoke template is the best way for now.