Write some doc about PeerConnection

Bug: webrtc:12738
Change-Id: I2a2c627ebe371a5ebd4c8e860d121a4ab8b2d291
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217680
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33944}

g3doc/sitemap.md

@@ -26,7 +26,7 @@
         *   [Video coding](/modules/video_coding/g3doc/index.md)
         *   [Stats](/video/g3doc/stats.md)
     *   DataChannel
-    *   PeerConnection
+    *   [PeerConnection](/pc/g3doc/peer_connection.md)
     *   Desktop capture
     *   Stats
 *   Testing

pc/g3doc/peer_connection.md

@@ -0,0 +1,59 @@
+<?% config.freshness.reviewed = '2021-05-07' %?>
+<?% config.freshness.owner = 'hta' %?>
+
+# PeerConnection and friends
+
+The PeerConnection is the C++-level implementation of the Javascript
+object "RTCPeerConnection" from the
+[WEBRTC specification](https://w3c.github.io/webrtc-pc/).
+
+Like many objects in WebRTC, the PeerConnection is used via a factory and an
+observer:
+
+ * PeerConnectionFactory, which is created via a static Create method and takes
+   a PeerConnectionFactoryDependencies structure listing such things as
+   non-default threads and factories for use by all PeerConnections using
+   the same factory. (Using more than one factory should be avoided, since
+   it takes more resources.)
+ * PeerConnection itself, which is created by the method called
+   PeerConnectionFactory::CreatePeerConnectionOrError, and takes a
+   PeerConnectionInterface::RTCConfiguration argument, as well as
+   a PeerConnectionDependencies (even more factories, plus other stuff).
+ * PeerConnectionObserver (a member of PeerConnectionDependencies), which
+   contains the functions that will be called on events in the PeerConnection
+
+These types are visible in the API.
+
+## Internal structure of PeerConnection and friends
+
+The PeerConnection is, to a large extent, a "God object" - most things
+that are done in WebRTC require a PeerConnection.
+
+Internally, it is divided into several objects, each with its own
+responsibilities, all of which are owned by the PeerConnection and live
+as long as the PeerConnection:
+
+ * SdpOfferAnswerHandler takes care of negotiating configurations with
+   a remote peer, using SDP-formatted descriptions.
+ * RtpTransmissionManager takes care of the lists of RtpSenders,
+   RtpReceivers and RtpTransceivers that form the heart of the transmission
+   service.
+ * DataChannelController takes care of managing the PeerConnection's
+   DataChannels and its SctpTransport.
+ * JsepTransportController takes care of configuring the details of senders
+   and receivers.
+ * Call does management of overall call state.
+ * RtcStatsCollector (and its obsolete sibling, StatsCollector) collects
+   statistics from all the objects comprising the PeerConnection when
+   requested.
+
+There are a number of other smaller objects that are also owned by
+the PeerConnection, but it would take too much space to describe them
+all here; please consult the .h files.
+
+PeerConnectionFactory owns an object called ConnectionContext, and a
+reference to this is passed to each PeerConnection. It is referenced
+via an rtc::scoped_refptr, which means that it is guaranteed to be
+alive as long as either the factory or one of the PeerConnections
+is using it.
+