Docs updates.

Change-Id: Iff395e1c6d81a484a2c4ad6e88357c0134971b78
diff --git a/libs/hwui/Snapshot.h b/libs/hwui/Snapshot.h
index ebedf53..062c986 100644
--- a/libs/hwui/Snapshot.h
+++ b/libs/hwui/Snapshot.h
@@ -116,6 +116,11 @@
     bool clipTransformed(const Rect& r, SkRegion::Op op = SkRegion::kIntersect_Op) {
         bool clipped = false;
 
+        // NOTE: The unimplemented operations require support for regions
+        // Supporting regions would require using a stencil buffer instead
+        // of the scissor. The stencil buffer itself is not too expensive
+        // (memory cost excluded) but on fillrate limited devices, managing
+        // the stencil might have a negative impact on the framerate.
         switch (op) {
             case SkRegion::kDifference_Op:
                 break;