improv(menu): add documentation.

src/widget/menu/action.rs

@@ -1,7 +1,57 @@
 use crate::widget::segmented_button::Entity;
 
+/// `MenuAction` is a trait that represents an action in a menu.
+///
+/// It is used to define the behavior of menu items when they are activated.
+/// Each menu item can have a unique action associated with it.
+///
+/// This trait is generic over a type `Message` which is the type of message
+/// that will be produced when the action is triggered.
+///
+/// # Example
+///
+/// ```
+/// use cosmic::widget::menu::action::MenuAction;
+/// use cosmic::widget::segmented_button::Entity;
+///
+/// #[derive(Clone, Copy, Eq, PartialEq)]
+/// enum MyMessage {
+///     Open,
+///     Save,
+///     Quit,
+/// }
+///
+/// #[derive(Clone, Copy, Eq, PartialEq)]
+/// enum MyAction {
+///     Open,
+///     Save,
+///     Quit,
+/// }
+///
+/// impl MenuAction for MyAction {
+///     type Message = MyMessage;
+///
+///     fn message(&self, entity: Option<Entity>) -> Self::Message {
+///         match self {
+///             MyAction::Open => MyMessage::Open,
+///             MyAction::Save => MyMessage::Save,
+///             MyAction::Quit => MyMessage::Quit,
+///         }
+///     }
+/// }
+/// ```
 pub trait MenuAction: Clone + Copy + Eq + PartialEq {
+    /// The type of message that will be produced when the action is triggered.
     type Message;
 
+    /// Returns a message of type `Self::Message` when the action is triggered.
+    ///
+    /// # Arguments
+    ///
+    /// * `entity` - An optional `Entity` that may be associated with the action.
+    ///
+    /// # Returns
+    ///
+    /// * `Self::Message` - The message that is produced when the action is triggered.
     fn message(&self, entity: Option<Entity>) -> Self::Message;
 }