Update default shaders in documentation of webgl classes

src/ol/webgl/PostProcessingPass.js

@@ -55,6 +55,10 @@ const DEFAULT_FRAGMENT_SHADER = `
  * This class is used to define Post Processing passes with custom shaders and uniforms.
  * This is used internally by {@link module:ol/webgl/Helper~WebGLHelper}.
  *
+ * Please note that the final output on the DOM canvas is expected to have premultiplied alpha, which means that
+ * a pixel which is 100% red with an opacity of 50% must have a color of (r=0.5, g=0, b=0, a=0.5).
+ * Failing to provide pixel colors with premultiplied alpha will result in render anomalies.
+ *
  * Default shaders are shown hereafter:
  *
  * * Vertex shader:
@@ -87,6 +91,7 @@ const DEFAULT_FRAGMENT_SHADER = `
  *
  *   void main() {
  *     gl_FragColor = texture2D(u_image, v_texCoord);
+ *     gl_FragColor.rgb *= gl_FragColor.a;
  *   }
  *   ```
  *