Attempt to update Xfermode comments to have parallel structure.

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index 0880f1403c6397e99ad783e3b647552f94256cc0..cb9557f92c28dec7dd05ac3d3f8b70ef7e4c74e0 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -70,15 +70,15 @@ public:
         kClear_Mode,    //!< [0, 0]
         kSrc_Mode,      //!< [Sa, Sc]
         kDst_Mode,      //!< [Da, Dc]
-        kSrcOver_Mode,  //!< [Sa + Da - Sa*Da, Rc = Sc + (1 - Sa)*Dc]
-        kDstOver_Mode,  //!< [Sa + Da - Sa*Da, Rc = Dc + (1 - Da)*Sc]
+        kSrcOver_Mode,  //!< [Sa + Da * (1 - Sa), Sc + Dc * (1 - Sa)]
+        kDstOver_Mode,  //!< [Da + Sa * (1 - Da), Dc + Sc * (1 - Da)]
         kSrcIn_Mode,    //!< [Sa * Da, Sc * Da]
-        kDstIn_Mode,    //!< [Sa * Da, Sa * Dc]
+        kDstIn_Mode,    //!< [Da * Sa, Dc * Sa]
         kSrcOut_Mode,   //!< [Sa * (1 - Da), Sc * (1 - Da)]
         kDstOut_Mode,   //!< [Da * (1 - Sa), Dc * (1 - Sa)]
-        kSrcATop_Mode,  //!< [Da, Sc * Da + (1 - Sa) * Dc]
-        kDstATop_Mode,  //!< [Sa, Sa * Dc + Sc * (1 - Da)]
-        kXor_Mode,      //!< [Sa + Da - 2 * Sa * Da, Sc * (1 - Da) + (1 - Sa) * Dc]
+        kSrcATop_Mode,  //!< [Da, Sc * Da + Dc * (1 - Sa)]
+        kDstATop_Mode,  //!< [Sa, Dc * Sa + Sc * (1 - Da)]
+        kXor_Mode,      //!< [Sa + Da - 2 * Sa * Da, Sc * (1 - Da) + Dc * (1 - Sa)]
         kPlus_Mode,     //!< [Sa + Da, Sc + Dc]
         kModulate_Mode, // multiplies all components (= alpha and color)