treewide: add defaultText (manual)

This enables generating module option documentation.

Add `defaultText` to options where the default value can't be
evaluated during metadata generation.

modules/nix-bitcoin.nix

@@ -7,17 +7,20 @@ with lib;
       pkgs = mkOption {
         type = types.attrs;
         default = (import ../pkgs { inherit pkgs; }).modulesPkgs;
+        defaultText = "nix-bitcoin/pkgs.modulesPkgs";
       };
 
       lib = mkOption {
         readOnly = true;
         default = import ../pkgs/lib.nix lib pkgs;
+        defaultText = "nix-bitcoin/pkgs/lib.nix";
       };
 
       torClientAddressWithPort = mkOption {
         readOnly = true;
         default = with config.services.tor.client.socksListenAddress;
           "${addr}:${toString port}";
+        defaultText = "(See source)";
       };
 
       # Torify binary that works with custom Tor SOCKS addresses
@@ -29,6 +32,7 @@ with lib;
             --address ${config.services.tor.client.socksListenAddress.addr} \
             "$@"
         '';
+        defaultText = "(See source)";
       };
 
       # A helper for using doas instead of sudo when doas is enabled
@@ -38,6 +42,7 @@ with lib;
                   # TODO-EXTERNAL: Use absolute path until https://github.com/NixOS/nixpkgs/pull/133622 is available.
                   then "/run/wrappers/bin/doas -u"
                   else "sudo -u";
+        defaultText = "(See source)";
       };
     };
   };