📝 Simpilify the structure of API references.

docs/api/menu-item.md

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+# menu-item
+
+## Class: MenuItem
+
+### new MenuItem(options)
+
+* `options` Object
+  * `click` Function - Callback when the menu item is clicked
+  * `selector` String - Call the selector of first responder when clicked (OS
+     X only)
+  * `type` String - Can be `normal`, `separator`, `submenu`, `checkbox` or
+     `radio`
+  * `label` String
+  * `sublabel` String
+  * `accelerator` String - In the form of `Command+R`, `Ctrl+C`,
+    `Shift+Command+D`, `D`, etc.
+  * `enabled` Boolean
+  * `visible` Boolean
+  * `checked` Boolean
+  * `groupId` Boolean - Should be specified for `radio` type menu item
+  * `submenu` Menu - Should be specified for `submenu` type menu item, when
+     it's specified the `type: 'submenu'` can be omitted for the menu item
+
+## Notes on accelerator
+
+On Linux and Windows, the `Command` key would not have any effect, you can
+use `CommandOrControl` which represents `Command` on OS X and `Control` on
+Linux and Windows to define some accelerators, you can also use its short
+alias `CmdOrCtrl`.