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/tasks/snapraid.nix

@@ -6,7 +6,7 @@ let cfg = config.snapraid;
 in
 {
   options.snapraid = with types; {
-    enable = mkEnableOption "SnapRAID";
+    enable = mkEnableOption (lib.mdDoc "SnapRAID");
     dataDisks = mkOption {
       default = { };
       example = {