update docs for backdrop

Bug: skia:
Change-Id: Ic3f7ad4b730d6aa616ec75dd6179d96327716b59
Reviewed-on: https://skia-review.googlesource.com/c/194003
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed