add preamble text to each service-providing-module

modules/ppp/default.nix

@@ -1,3 +1,12 @@
+## PPP
+## ===
+##
+## A rudimentary PPPoE (PPP over Ethernet) configuration to address
+## the case where your Liminix device is connected to an upstream
+## network using PPPoE. This is typical for UK broadband connections
+## (except "cable"), and common in some other localities as well: ask
+## your ISP if this is you.
+
 { lib, pkgs, config, ...}:
 let
   inherit (lib) mkOption types;