Work to improve blendmode dox

Change-Id: I0ea12ad5c651aa6bd2f9b22c1ad539415d5da9bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367760
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed