make explicit the requirement that all colorfilters are reentrant-safe

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index ed07bd5821c320b868884871938151e08ee60f53..edcb9fb9be7f2892d32987f1a2cc1d52a76be463 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -25,7 +25,10 @@ class SkString;
     can be called to return an instance of any of the predefined subclasses as
     specified in the Modes enum. When an SkXfermode is assigned to an SkPaint,
     then objects drawn with that paint have the xfermode applied.
-*/
+
+ *  All subclasses are required to be reentrant-safe : it must be legal to share
+ *  the same instance between several threads.
+ */
 class SK_API SkXfermode : public SkFlattenable {
 public:
     SK_DECLARE_INST_COUNT(SkXfermode)