Ask HN: Tools to generate coverage of user documentation for code

Does anyone know of tools/techniques for generating a coverage report of user documentation over source code? In other words, I'd like to be able to automatically determine when there is documentation missing for an implemented feature or if there is documentation for a feature that was removed.

I'm asking because I've been working on a tool for many years and it's grown large enough that I have a hard time keeping track of what all the parts do and what has and has not been documented. What I'm thinking I'd like to do is add a tag to a piece of source code (C++) and put the corresponding tag in the user docs (RST). Then, during the build, I'd like a report of untagged code, code/docs with matching tags, and code/docs with unmatched tags.

I also feel like there should be at least two levels of documentation to be tracked: 1) the overall feature and 2) fine-details about how the feature behaves under different circumstances. For example, a feature that interacted with the network would have an overall description and some extra notes about how timeouts/retries are handled.

Does anyone have any experience with such a thing or have a similar interest?