make explicit the requirement that all colorfilters are reentrant-safe

include/core/SkColorFilter.h

Index: include/core/SkColorFilter.h
diff --git a/include/core/SkColorFilter.h b/include/core/SkColorFilter.h
index 6359563d5180b6b35b0540fde43770fd79eb0779..46ecedc6dfc5836dfe18097c47112e64a9277922 100644
--- a/include/core/SkColorFilter.h
+++ b/include/core/SkColorFilter.h
@@ -18,6 +18,14 @@ class SkBitmap;
 class GrEffectRef;
 class GrContext;
 
+/**
+ *  ColorFilters are optional objects in the drawing pipeline. When present in
+ *  a paint, they are called with the "src" colors, and return new colors, which
+ *  are then passed onto the next stage (either ImageFilter or Xfermode).
+ *
+ *  All subclasses are required to be reentrant-safe : it must be legal to share
+ *  the same instance between several threads.
+ */
 class SK_API SkColorFilter : public SkFlattenable {
 public:
     SK_DECLARE_INST_COUNT(SkColorFilter)