nixos: correct improper uses of mkEnableOption, clarify service descriptions

Several service definitions used `mkEnableOption` with text starting
with "Whether to", which produced funny option descriptions like
"Whether to enable Whether to run the rspamd daemon..".

This commit corrects this, and adds short descriptions of services
to affected service definitions.

nixos/modules/services/networking/pptpd.nix

@@ -5,7 +5,7 @@ with lib;
 {
   options = {
     services.pptpd = {
-      enable = mkEnableOption "Whether pptpd should be run on startup.";
+      enable = mkEnableOption "pptpd, the Point-to-Point Tunneling Protocol daemon";
 
       serverIp = mkOption {
         type        = types.string;