adding note about cumulative mask property to mask closes #5012

Author: lmccart

src/image/p5.Image.js

@@ -603,7 +603,8 @@ p5.Image.prototype.copy = function(...args) {
 /**
  * Masks part of an image from displaying by loading another
  * image and using its alpha channel as an alpha channel for
- * this image.
+ * this image. Masks are cumulative, one applied to an image
+ * object, they cannot be removed.
  *
  * @method mask
  * @param {p5.Image} srcImage source image