Quoting Miles Bader <mi...@gnu.org>:
With elisp, I've found that in practice I usually start by copying the docstring (the "in code doc") to the manual (the "doc doc"), but almost always end up largely rewriting to fit the context in the manual better, and to explain things in more detail (modern docstrings tend to be rather verbose compared to docstrings-of-old, but they're still generally more terse than the manual).
Still, if anything copyrightable is left of the copied text, you need license compatibility (or full copyright to the original text) in order to be able to publish the result. I find that a similar process is also often natural with going from code comments to gcc internals documentation, but if I don't have copyright to the comment, I don't want to deal with the license problems, so I rather leave the documentation for someone else (or nobody...) to write.
