Add blit shader comment (#3301)

wgpu/examples/mipmap/blit.wgsl

@@ -3,6 +3,24 @@ struct VertexOutput {
  @location(0) tex_coords: vec2<f32>,
 };
 
+// meant to be called with 3 vertex indices: 0, 1, 2
+// draws one large triangle over the clip space like this:
+// (the asterisks represent the clip space bounds)
+//-1,1 1,1
+// ---------------------------------
+// | * .
+// | * .
+// | * .
+// | * .
+// | * . 
+// | * .
+// |***************
+// | . 1,-1 
+// | .
+// | .
+// | .
+// | .
+// |.
 @vertex
 fn vs_main(@builtin(vertex_index) vertex_index: u32) -> VertexOutput {
  var result: VertexOutput;