Add class comments to Call and ChannelInterface

These class comments can help people to keep the relationship between
various classes / levels of abstraction straight.

Bug: None
Change-Id: I9fc899ac9c1080a406b06e88a38883cc3f00093d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/248169
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35769}

call/call.h

@@ -64,9 +64,16 @@ class SharedModuleThread final {
   mutable std::unique_ptr<Impl> impl_;
 };
 
+// A Call represents a two-way connection carrying zero or more outgoing
+// and incoming media streams, transported over one or more RTP transports.
+
 // A Call instance can contain several send and/or receive streams. All streams
 // are assumed to have the same remote endpoint and will share bitrate estimates
 // etc.
+
+// When using the PeerConnection API, there is an one to one relationship
+// between the PeerConnection and the Call.
+
 class Call {
  public:
   using Config = CallConfig;

pc/channel_interface.h

@@ -32,7 +32,14 @@ class VideoChannel;
 class VoiceChannel;
 struct MediaConfig;
 
-// ChannelInterface contains methods common to voice, video and data channels.
+// A Channel is a construct that groups media streams of the same type
+// (audio or video), both outgoing and incoming.
+// When the PeerConnection API is used, a Channel corresponds one to one
+// to an RtpTransceiver.
+// When Unified Plan is used, there can only be at most one outgoing and
+// one incoming stream. With Plan B, there can be more than one.
+
+// ChannelInterface contains methods common to voice and video channels.
 // As more methods are added to BaseChannel, they should be included in the
 // interface as well.
 class ChannelInterface {