Update documentation for getData methods in VideoFrame.I420Buffer.
Update documentation for getData methods in VideoFrame.I420Buffer.

Bug: webrtc:7749
Change-Id: I8151c9e102340e10d13b3fb946ec5ce307b139b3
No-Try: True
TBR: 9507adee858471ab168c339dc72b814b168c081b@webrtc.org
Reviewed-on: https://webrtc-review.googlesource.com/4020
Commit-Queue: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Reviewed-by: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19993}
ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc