[RenderScript] Update documentation for Allocation.setAutoPadding().

Bug: 26862970
Change-Id: Iaa88ac0266e5d2cac2837f1d13353aacc6177274
(cherry picked from commit 9ee7607615)

rs/java/android/renderscript/Allocation.java

@@ -296,8 +296,13 @@ public class Allocation extends BaseObj {
     }
 
     /**
-     * Enable/Disable AutoPadding for Vec3 elements.
-     * By default: Diabled.
+     * Enable/Disable AutoPadding for Vec3 Elements.
+     *
+     * <p> Vec3 Elements, such as {@link Element#U8_3} are treated as Vec4 Elements
+     * with the fourth vector element used as padding. Enabling the AutoPadding feature
+     * will automatically add/remove the padding when you copy to/from an Allocation
+     * with a Vec3 Element.
+     * <p> By default: Disabled.
      *
      * @param useAutoPadding True: enable AutoPadding; False: disable AutoPadding
      *