[Proposal] Improve Canvas Documentation (flutter/engine#30069)

For clarity, the text has been improved over when it was suggested.

Fixes https://github.com/flutter/flutter/issues/93190

engine/src/flutter/lib/ui/painting.dart

@@ -3928,12 +3928,20 @@ class Vertices extends NativeFieldWrapperClass1 {
   /// Creates a set of vertex data for use with [Canvas.drawVertices].
   ///
   /// The [mode] and [positions] parameters must not be null.
+  /// The [positions] parameter is a list of triangular mesh vertices(xy).
   ///
   /// If the [textureCoordinates] or [colors] parameters are provided, they must
   /// be the same length as [positions].
   ///
+  /// The [textureCoordinates] parameter is used to cutout
+  /// the image set in the image shader.
+  /// The cut part is applied to the triangular mesh.
+  /// Note that the [textureCoordinates] are the coordinates on the image.
+  ///
   /// If the [indices] parameter is provided, all values in the list must be
   /// valid index values for [positions].
+  ///
+  /// e.g. The [indices] parameter for a simple triangle is [0,1,2].
   Vertices(
     VertexMode mode,
     List<Offset> positions, {
@@ -4674,6 +4682,7 @@ class Canvas extends NativeFieldWrapperClass1 {
   ///   * [new Vertices], which creates a set of vertices to draw on the canvas.
   ///   * [Vertices.raw], which creates the vertices using typed data lists
   ///     rather than unencoded lists.
+  ///   * [paint], Image shaders can be used to draw images on a triangular mesh.
   void drawVertices(Vertices vertices, BlendMode blendMode, Paint paint) {
 
     assert(vertices != null); // vertices is checked on the engine side