Update comments about blending effect in DecompositeEffect()

Hopefully the new comments are more understandable.

Change-Id: I26e23f078ca9f15a371f2ac8eccd281c2435551c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348939


Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118903}