I recall, back in the AmigaOS days, we kept the documentation inline with the code. E.g. you had your exported API function in a shared library code's .c file, with the documentation right above it sitting in a special formatted comment. Easily kept in sync (code wasn't that convoluted back then either.) Afterwards, during the build phase, the documentation (for other developers) was extracted on-the-fly. There was a fixed format/style and the documentation was compact and actually useful too.

(aka "autodocs", for those who remember the term)