fix(docgen): root_dir doc shows paths from CI env

[justinmk]

Problem:

When root_dir is not defined as a function, the generated docs show the paths resolved in the CI env:

root_dir : lua "/home/runner/work/nvim-lspconfig/nvim-lspconfig"

Solution:

Always show the "gF" message for root_dir, even if it is not a function.

Fix #3628

[github-actions]

Note that configs.md or configs.txt will be regenerated by the docgen CI process. Edit the Lua source file instead. For details on generating documentation, see: https://github.com/neovim/nvim-lspconfig/blob/master/CONTRIBUTING.md#generating-docs