Improve comments.

Use the current parameter names in the comment for SetNackMode().

Add a warning comment about the lifetime of the return value of
GetNackList().

R=stefan@webrtc.org
BUG=none
TEST=none

Review URL: https://webrtc-codereview.appspot.com/52599004

Cr-Commit-Position: refs/heads/master@{#9321}
1 file changed