docs: clarify config and opts relationship in examples

Author: zuqini

README.md

@@ -81,7 +81,11 @@ return {
   keys = {
     { '<leader>ff', function() require('telescope.builtin').find_files() end, desc = 'Find files' },
   },
-  opts = {}, -- automatically calls require(MAIN).setup(opts) when defined
+  opts = {},
+  -- config is executed when the plugin loads. The default implementation will automatically run require(MAIN).setup(opts) if opts or config = true is set.
+  config = function()
+    ...
+  end,
 }
 ```
 

doc/zpack.txt

@@ -120,6 +120,12 @@ Each file returns a spec or list of specs (see |zpack-examples| or
           desc = 'Find files' },
       },
       opts = {},
+      -- config is executed when the plugin loads.
+      -- The default implementation will automatically run
+      -- require(MAIN).setup(opts) if opts or config = true is set.
+      config = function()
+        ...
+      end,
     }
 <