Many of these interfaces are not intuitive, or are the way they are for
complex historical reasons, so it would be nice to document these things
for future developers.
Also, many nonstandard things (such as RTCConfiguration options) were
not documented at all before this CL.
BUG=webrtc:7131
TBR=pthatcher@webrtc.org
NOTRY=True
Review-Url: https://codereview.webrtc.org/2680273002
Cr-Commit-Position: refs/heads/master@{#16485}