update dox for dithering

Bug: skia:6604
Change-Id: I21b505c54c1d43abf62b7c577e7dd88e53ed30fd
Reviewed-on: https://skia-review.googlesource.com/16156
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/include/core/SkPaint.h b/include/core/SkPaint.h
index 0fc4f5c..43bacbe 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -100,7 +100,7 @@
     */
     enum Flags {
         kAntiAlias_Flag       = 0x01,   //!< mask to enable antialiasing
-        kDither_Flag          = 0x04,   //!< mask to enable dithering
+        kDither_Flag          = 0x04,   //!< mask to enable dithering. see setDither()
         kFakeBoldText_Flag    = 0x20,   //!< mask to enable fake-bold text
         kLinearText_Flag      = 0x40,   //!< mask to enable linear-text
         kSubpixelText_Flag    = 0x80,   //!< mask to enable subpixel text positioning
@@ -154,9 +154,12 @@
         return SkToBool(this->getFlags() & kDither_Flag);
     }
 
-    /** Helper for setFlags(), setting or clearing the kDither_Flag bit
-        @param dither   true to enable dithering, false to disable it
-        */
+    /**
+     *  Helper for setFlags(), setting or clearing the kDither_Flag bit
+     *  @param dither   true to enable dithering, false to disable it
+     *
+     *  Note: gradients ignore this setting and always dither.
+     */
     void setDither(bool dither);
 
     /** Helper for getFlags(), returning true if kLinearText_Flag bit is set