update dox for dithering

Bug: skia:6604
Change-Id: I21b505c54c1d43abf62b7c577e7dd88e53ed30fd
Reviewed-on: https://skia-review.googlesource.com/16156
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed