Two of the most successful programming languages of our time #Python & #Golang (yes and #Elisp!) have one thing in common: in-source docstrings.
They make it easy to document source code and thus make it more accessible to contributors. Languages that treat documentation as a separate issue are mostly tools for individuals with half-baked single-maintainer packages.
Even the most brilliant lang will have low adoption if documentation is not a big part of its core features.