modules/github-runner: Improve description of url

We spent a whole afternoon debugging this, because upstream has very
bad software quality and the error messages were incredibly
misleading.

So let’s document it for the sanity of other people.

Btw, I think the implementation of our module is pretty brittle,
especially the part about diffing tokens to check whether they
changed. We should rather just request a new builder registration
every time, it’s not that much overhead, and always set `replace` so
it is idempotent.

nixos/modules/services/continuous-integration/github-runner.nix

@@ -34,6 +34,14 @@ in
         Repository to add the runner to.
 
         Changing this option triggers a new runner registration.
+
+        IMPORTANT: If your token is org-wide (not per repository), you need to
+        provide a github org link, not a single repository, so do it like this
+        <literal>https://github.com/nixos</literal>, not like this
+        <literal>https://github.com/nixos/nixpkgs</literal>.
+        Otherwise, you are going to get a <literal>404 NotFound</literal>
+        from <literal>POST https://api.github.com/actions/runner-registration</literal>
+        in the configure script.
       '';
       example = "https://github.com/nixos/nixpkgs";
     };