**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.cc
int command_id,
                             const base::string16& label) {
  model_->InsertCheckItemAt(index, command_id, label);
}
void Menu::InsertRadioItemAt(int index,
                             int command_id,
                             const base::string16& label,
                             int group_id) {
  model_->InsertRadioItemAt(index, command_id, label, group_id);
}
void Menu::InsertSubMenuAt(int index,
                           int command_id,
                           const base::string16& label,
                           Menu* menu) {
  menu->parent_ = this;
  model_->InsertSubMenuAt(index, command_id, label, menu->model_.get());
}
void Menu::SetIcon(int index, const gfx::Image& image) {
  model_->SetIcon(index, image);
}
void Menu::SetSublabel(int index, const base::string16& sublabel) {
  model_->SetSublabel(index, sublabel);
}
void Menu::SetRole(int index, const base::string16& role) {
  model_->SetRole(index, role);
}
void Menu::Clear() {
  model_->Clear();
}
int Menu::GetIndexOfCommandId(int command_id) {