modules/github-runner: Improve tokenFile docs

We’ve been having trouble figuring out which kind of token to use and
why our setup would break every few system updates.

This should clarify which options there are, and which ones lead to
better results.

Ideally there would be a manual section that has a step-by-step guide
on how to set up the github runner, with screenshots and everything.

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

@@ -41,17 +41,42 @@ with lib;
   tokenFile = mkOption {
     type = types.path;
     description = lib.mdDoc ''
-      The full path to a file which contains either a runner registration token or a
-      (fine-grained) personal access token (PAT).
-      The file should contain exactly one line with the token without any newline.
-      If a registration token is given, it can be used to re-register a runner of the same
-      name but is time-limited. If the file contains a PAT, the service creates a new
-      registration token on startup as needed. Make sure the PAT has a scope of
-      `admin:org` for organization-wide registrations or a scope of
-      `repo` for a single repository. Fine-grained PATs need read and write permission
-      to the "Administration" resources.
+      The full path to a file which contains either
 
-      Changing this option or the file's content triggers a new runner registration.
+      * a fine-grained personal access token (PAT),
+      * a classic PAT
+      * or a runner registration token
+
+      Changing this option or the `tokenFile`’s content triggers a new runner registration.
+
+      We suggest using the fine-grained PATs. A runner registration token is valid
+      only for 1 hour after creation, so the next time the runner configuration changes
+      this will give you hard-to-debug HTTP 404 errors in the configure step.
+
+      The file should contain exactly one line with the token without any newline.
+      (Use `echo -n '…token…' > …token file…` to make sure no newlines sneak in.)
+
+      If the file contains a PAT, the service creates a new registration token
+      on startup as needed.
+      If a registration token is given, it can be used to re-register a runner of the same
+      name but is time-limited as noted above.
+
+      For fine-grained PATs:
+
+      Give it "Read and Write access to organization/repository self hosted runners",
+      depending on whether it is organization wide or per-repository. You might have to
+      experiment a little, fine-grained PATs are a `beta` Github feature and still subject
+      to change; nonetheless they are the best option at the moment.
+
+      For classic PATs:
+
+      Make sure the PAT has a scope of `admin:org` for organization-wide registrations
+      or a scope of `repo` for a single repository.
+
+      For runner registration tokens:
+
+      Nothing special needs to be done, but updating will break after one hour,
+      so these are not recommended.
     '';
     example = "/run/secrets/github-runner/nixos.token";
   };