Write documentation for the new canvas API

wgpu/src/widget/canvas/geometry.rs

@@ -1,5 +1,13 @@
 use crate::Primitive;
 
+/// A bunch of shapes that can be drawn.
+///
+/// [`Geometry`] can be easily generated with a [`Frame`] or stored in a
+/// [`Cache`].
+///
+/// [`Geometry`]: struct.Geometry.html
+/// [`Frame`]: struct.Frame.html
+/// [`Cache`]: struct.Cache.html
 #[derive(Debug, Clone)]
 pub struct Geometry(Primitive);
 
@@ -8,6 +16,12 @@ impl Geometry {
         Self(primitive)
     }
 
+    /// Turns the [`Geometry`] into a [`Primitive`].
+    ///
+    /// This can be useful if you are building a custom widget.
+    ///
+    /// [`Geometry`]: struct.Geometry.html
+    /// [`Primitive`]: ../enum.Primitive.html
     pub fn into_primitive(self) -> Primitive {
         self.0
     }