add descriptions for extraPlugins

lib/types/plugins.nix

@@ -107,10 +107,13 @@ with lib; let
         after = mkOption {
           type = listOf str;
           default = [];
+          description = "Setup this plugin after the following ones.";
         };
         setup = mkOption {
           type = lines;
           default = "";
+          description = "Lua code to run during setup.";
+          example = "require('aerial').setup {}";
         };
       };
     };

modules/core/default.nix

@@ -161,6 +161,22 @@ in {
     extraPlugins = mkOption {
       type = types.attrsOf nvim.types.extraPluginType;
       default = {};
+      description = ''
+        List of plugins and related config.
+        Note that these are setup after builtin plugins.
+      '';
+      example = literalExpression ''
+          with pkgs.vimPlugins; {
+          aerial = {
+            package = aerial-nvim;
+            setup = "require('aerial').setup {}";
+          };
+          harpoon = {
+            package = harpoon;
+            setup = "require('harpoon').setup {}";
+            after = ["aerial"];
+          };
+        }'';
     };
 
     globals = mkOption {