docs: 📝 add PAM fingerprint configuration guidance for T14 Gen 6

Add comprehensive comments explaining how to configure PAM services
for complete fingerprint authentication in GNOME, including warnings
about potential login issues and step-by-step setup instructions.

Author: bashfulrobot

lenovo/thinkpad/t14/intel/gen6/default.nix

@@ -35,4 +35,26 @@
   # NOTE: PAM configuration should be done in host config to avoid login issues
   # See: https://github.com/NixOS/nixpkgs/issues/171136
   services.fprintd.enable = lib.mkDefault true;
+
+  # For complete fingerprint authentication in GNOME, add the following to your host config:
+  #
+  # security.pam.services = {
+  #   # Enable fingerprint authentication for sudo
+  #   sudo.fprintAuth = lib.mkDefault true;
+  #   
+  #   # Enable fingerprint authentication for su
+  #   su.fprintAuth = lib.mkDefault true;
+  #   
+  #   # Enable fingerprint authentication for screen unlock
+  #   xscreensaver.fprintAuth = lib.mkDefault true;
+  #   
+  #   # WARNING: login.fprintAuth may break GDM password authentication
+  #   # Only enable if you understand the risks:
+  #   # login.fprintAuth = lib.mkDefault true;
+  # };
+  #
+  # After configuration:
+  # 1. Rebuild your system
+  # 2. Enroll fingerprint: sudo fprintd-enroll $USER
+  # 3. Test sudo and screen unlock with fingerprint
 }