udpate dox for colorfilter fragment processor requirements

include/core/SkColorFilter.h

Index: include/core/SkColorFilter.h
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index 14d33069f01ae9a74c2ae04d15db5758a20ef58d..5a25814c6ba854932bc007f637708172e0486a8b 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -144,6 +144,9 @@ public:
      *  on them when they are finished. If more than one processor is appended, they will be
      *  applied in FIFO order.
      *
+     *  The fragment processor(s) must each return their color as a premul normalized value
+     *  e.g. each component between [0..1] and each color component <= alpha.
+     *
      *  If the subclass returns false, then it should not modify the array at all.
      */
     virtual bool asFragmentProcessors(GrContext*, SkTDArray<GrFragmentProcessor*>*) const {