Update test spec.

Change-Id: I045b445c0f0c9b524cdce6327826068581df8082
diff --git a/doc/testspecs/GLES31/functional.blend_equation_advanced.txt b/doc/testspecs/GLES31/functional.blend_equation_advanced.txt
index 72d06ff..cc7822d 100644
--- a/doc/testspecs/GLES31/functional.blend_equation_advanced.txt
+++ b/doc/testspecs/GLES31/functional.blend_equation_advanced.txt
@@ -32,6 +32,7 @@
 Excludes (will be added later):
  + Use of blend_support_all qualifier
  + Interaction with other per-fragment operations
+ + Non-premultiplied / invalid color inputs
  + Negative tests
 
 Description: