With Ignition Extensions installed, you could call system.project.getProject() to get an instance of the Project Java class. From there, call getResourcesOfType with a resource type of com.inductiveautomation.factorypmi.common.VisionModule.WINDOW_RESOURCE (this'll only work directly in the designer; doing it from the gateway scope will be a bit trickier).
That will give you a list of ProjectResources, which have a getDocumentation method.
Technically, Ignition Extensions is not required (you can access the project through other scripting means) but it's the easiest entrypoint.