Update documentation for getData methods in VideoFrame.I420Buffer.

Bug: webrtc:7749
Change-Id: I8151c9e102340e10d13b3fb946ec5ce307b139b3
No-Try: True
TBR: magjed@webrtc.org
Reviewed-on: https://webrtc-review.googlesource.com/4020
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19993}

sdk/android/api/org/webrtc/VideoFrame.java

@@ -57,8 +57,20 @@ public class VideoFrame {
    * Interface for I420 buffers.
    */
   public interface I420Buffer extends Buffer {
+    /**
+     * Returns a direct ByteBuffer containing Y-plane data. The buffer size is at least getStrideY()
+     * * getHeight() bytes.
+     */
     ByteBuffer getDataY();
+    /**
+     * Returns a direct ByteBuffer containing U-plane data. The buffer size is at least getStrideU()
+     * * ((getHeight() + 1) / 2) bytes.
+     */
     ByteBuffer getDataU();
+    /**
+     * Returns a direct ByteBuffer containing V-plane data. The buffer size is at least getStrideV()
+     * * ((getHeight() + 1) / 2) bytes.
+     */
     ByteBuffer getDataV();
 
     int getStrideY();