Document missing 8bit blend stages

Change-Id: Id626f954fe45546a015a1bd423f19cca5f8967a9
Reviewed-on: https://skia-review.googlesource.com/37861
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/jumper/SkJumper_stages_8bit.cpp b/src/jumper/SkJumper_stages_8bit.cpp
index 1d6bc8a..5ee3417 100644
--- a/src/jumper/SkJumper_stages_8bit.cpp
+++ b/src/jumper/SkJumper_stages_8bit.cpp
@@ -440,3 +440,11 @@
              + if_then_else(d*2 <= da, s*d*2, sa*da - (da - d)*(sa - s)*2);
     });
 }
+
+// Missing blendmode specializations:
+//
+//   plus        - tricky due to clamping vs. partial coverage (especially with LCD masks).
+//
+//   colorburn  |
+//   colordodge  > these involve division, which makes them (much) slower than the float stages.
+//   softlight  |