Improve comments.
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9321}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 603175a395f8b00bd20c5fd46c044d23a18f64cf

Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,aleonliao/webrtc-trunk,sippet/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,sippet/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,sippet/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk