I'm doing it using an internal macro `@docref` [1] to track the line number and then in `src/doc.jl` I store the documentation in `__METADATA__` along with the line and source file found using `@docref`. A bit hacky, but it's only for a couple of docs.
[1] https://github.com/MichaelHatherly/Docile.jl/blob/c82675d4a39932d1d378e954844018cefc091858/src/Docile.jl#L14-L17
