I've poured a lot of time and effort into documenting my C++ TOML library, generally seeking to write the docs as I would wish to find them if I were a new user: https://marzer.github.io/tomlplusplus/
The docs are built using a spin-off of the documentation generator for the magnum graphics engine, which are themselves a really good documentation example: https://magnum.graphics/
I actually tried using toml11 before writing toml++, and my main gripe with it was the complexity. It does a _lot_, and I found it to be pretty unergonomic to actually use.
That, and it's not as configurable (e.g. no support for exception-less).
The docs are built using a spin-off of the documentation generator for the magnum graphics engine, which are themselves a really good documentation example: https://magnum.graphics/