nixos/*: automatically convert option descriptions

conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running

    nix-doc-munge nixos/**/*.nix
    nix-doc-munge --import nixos/**/*.nix

the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.

nixos/modules/programs/steam.nix

@@ -13,7 +13,7 @@ let
   };
 in {
   options.programs.steam = {
-    enable = mkEnableOption "steam";
+    enable = mkEnableOption (lib.mdDoc "steam");
 
     remotePlay.openFirewall = mkOption {
       type = types.bool;