Write documentation for new pane_grid API

native/src/widget/pane_grid/content.rs

@@ -1,12 +1,30 @@
 use crate::pane_grid::Axis;
 
+/// The content of a [`PaneGrid`].
+///
+/// [`PaneGrid`]: struct.PaneGrid.html
 #[derive(Debug, Clone)]
 pub enum Content<T> {
+    /// A split of the available space.
     Split {
+        /// The direction of the split.
         axis: Axis,
+
+        /// The ratio of the split in [0.0, 1.0].
         ratio: f32,
+
+        /// The left/top [`Content`] of the split.
+        ///
+        /// [`Content`]: enum.Node.html
         a: Box<Content<T>>,
+
+        /// The right/bottom [`Content`] of the split.
+        ///
+        /// [`Content`]: enum.Node.html
         b: Box<Content<T>>,
     },
+    /// A [`Pane`].
+    ///
+    /// [`Pane`]: struct.Pane.html
     Pane(T),
 }