nixos/nix-flakes: ref to man page about flake registry (#400770)

nixos/modules/config/nix-flakes.nix

@@ -19,6 +19,10 @@ let
 
   cfg = config.nix;
 
+  flakeRefFormat = ''
+    The format of flake references is described in {manpage}`nix3-flake(1)`.
+  '';
+
 in
 {
   options = {
@@ -46,7 +50,11 @@ in
                     type = "indirect";
                     id = "nixpkgs";
                   };
-                  description = "The flake reference to be rewritten.";
+                  description = ''
+                    The flake reference to be rewritten.
+
+                    ${flakeRefFormat}
+                  '';
                 };
                 to = mkOption {
                   type = referenceAttrs;
@@ -55,7 +63,11 @@ in
                     owner = "my-org";
                     repo = "my-nixpkgs";
                   };
-                  description = "The flake reference {option}`from` is rewritten to.";
+                  description = ''
+                    The flake reference {option}`from` is rewritten to.
+
+                    ${flakeRefFormat}
+                  '';
                 };
                 flake = mkOption {
                   type = types.nullOr types.attrs;
@@ -96,6 +108,8 @@ in
         default = { };
         description = ''
           A system-wide flake registry.
+
+          See {manpage}`nix3-registry(1)` for more information.
         '';
       };
     };