So the issue here is that markdownlint-cli2 is meant to lint globs, not single files (see the documentation). The user either needs to specify a globs property in their config file or add an argument in their null-ls config that specifies a glob to be linted, e.g. args = { "**/*.md" }. Can we update the documentation accordingly?

Sure, I didn't have a lot of experience with this.

Would you like me to open an issue upstream?

I think it's working as expected, so I just mean in terms of updating our documentation here. If you look at the built-in's source, you'll see that we have a field where we can inject Markdown that will then be output into this file. We can then tell users about the specific configuration requirements for this source.

So are you saying:

local null_ls = require('null-ls')
null_ls.setup {
  sources = {
    null_ls.builtins.diagnostics.markdownlint_cli2.with {
      args = { "**/*.md" },
    },
  },
}

Is actually correct?

You could do that, or you could use globs a config file (which I imagine would be the preferred approach on non-solo projects). You can see this document for info on adding to the documentation.