Yes, I've checked the discussion and well, yes, now I see your point...
Well, indeed, there are several tools for documenting the code (like Doxygen, QDoc, STANDARDESE, etc. - for C++, to name a few).
However, they are rather oriented to documenting of the "source code" - and therefore, they use such notions as "class", "header", "function" etc. for making documentation.
Therefore, for producing the user manual - such a documentation on the source layer seems to be not (at least, directly) applicable. For example, some feature (or action) may involve lot's of files, functions, classes etc. and documenting them is too low-level and not informative for the end user.
Therefore, I'm afraid the only way to go there - is just to create manual documentation for features.
At least, right now I have no idea how to use the code documentation approach/tool to achieve some useful output...
With my merge request I usually add description for the added functionality (located here:
https://github.com/LibreCAD/LibreCAD/tree/master/ChangeLogs), yet still it's quite brief to be useful for the end user.