Added comments to Function

To be honest, I still don't understand exactly what's happening here. All I know is that it's at least somewhat understandable what's going on here.

rosidl_runtime_rs/src/sequence.rs

@@ -130,6 +130,17 @@ impl<T: Debug + SequenceAlloc> Debug for Sequence<T> {
 }
 
 impl<T: SequenceAlloc> Default for Sequence<T> {
+    /// Creates a new `Sequence` instance with default values.
+    ///
+    /// The default `Sequence` has the following properties:
+    ///
+    /// - `data` is set to a null pointer (`std::ptr::null_mut()`).
+    /// - `size` is set to 0.
+    /// - `capacity` is set to 0.
+    ///
+    /// # Returns
+    ///
+    /// A new `Sequence` instance with default values.
     fn default() -> Self {
         Self {
             data: std::ptr::null_mut(),