**Is your feature request related to a problem? Please describe.**

MenuItem labels are meant to be short. There's currently no place to elaborate/clarify them.

**Describe the solution you'd like**

`NSMenuItem` supports a [`toolTip`](https://developer.apple.com/documentation/appkit/nsmenuitem/1514848-tooltip?language=objc) property. Would be nice if Electron could expose this.

From what I could find, [GTK (Linux) does not have any way to set tooltip on menu items](https://developer.gnome.org/gtk3/stable/GtkMenuItem.html).

No idea about Windows.

**Describe alternatives you've considered**

None.

**Additional context**

No.
shell/browser/api/atom_api_menu.h
void OnMenuWillShow(ui::SimpleMenuModel* source) override;
  virtual void PopupAt(TopLevelWindow* window,
                       int x,
                       int y,
                       int positioning_item,
                       const base::Closure& callback) = 0;
  virtual void ClosePopupAt(int32_t window_id) = 0;
  std::unique_ptr<AtomMenuModel> model_;
  Menu* parent_ = nullptr;
  
  void OnMenuWillClose() override;
  void OnMenuWillShow() override;
 private:
  void InsertItemAt(int index, int command_id, const base::string16& label);
  void InsertSeparatorAt(int index);
  void InsertCheckItemAt(int index,
                         int command_id,
                         const base::string16& label);
  void InsertRadioItemAt(int index,
                         int command_id,
                         const base::string16& label,
                         int group_id);
  void InsertSubMenuAt(int index,
                       int command_id,
                       const base::string16& label,
                       Menu* menu);
  void SetIcon(int index, const gfx::Image& image);
  void SetSublabel(int index, const base::string16& sublabel);
  void SetRole(int index, const base::string16& role);
  void Clear();