Write documentation for new canvas module

wgpu/src/widget/canvas/layer.rs

@@ -1,16 +1,25 @@
-mod cached;
+//! Produce, store, and reuse geometry.
+mod cache;
 
-pub use cached::Cached;
+pub use cache::Cache;
 
-use crate::{canvas::Frame, triangle};
+use crate::triangle;
 
 use iced_native::Size;
 use std::sync::Arc;
 
+/// A layer that can be presented at a [`Canvas`].
+///
+/// [`Canvas`]: ../struct.Canvas.html
 pub trait Layer: std::fmt::Debug {
+    /// Draws the [`Layer`] in the given bounds and produces [`Mesh2D`] as a
+    /// result.
+    ///
+    /// The [`Layer`] may choose to store the produced [`Mesh2D`] locally and
+    /// only recompute it when the bounds change, its contents change, or is
+    /// otherwise explicitly cleared by other means.
+    ///
+    /// [`Layer`]: trait.Layer.html
+    /// [`Mesh2D`]: ../../../triangle/struct.Mesh2D.html
     fn draw(&self, bounds: Size) -> Arc<triangle::Mesh2D>;
 }
-
-pub trait Drawable {
-    fn draw(&self, frame: &mut Frame);
-}