Update MaterialType.canvas documentation.

While the documentation for MaterialType.canvas described it as
an infinite shape. The implementation has been clipping it to the
bounding rectangle for a while. And it is already used in the wild as a
rectangular piece. So we just update the documentation to match that.

packages/flutter/lib/src/material/material.dart

@@ -22,7 +22,7 @@ typedef Rect RectCallback();
 ///  * [Material], in particular [Material.type]
 ///  * [kMaterialEdges]
 enum MaterialType {
-  /// Infinite extent using default theme canvas color.
+  /// Rectangle using default theme canvas color.
   canvas,
 
   /// Rounded edges, card theme color.