Add connection event documentation.

Runtime/Connection/NetworkConnectionManager.cs

@@ -13,9 +13,21 @@ namespace Unity.Netcode
 
     public enum ConnectionEvent
     {
+        /// <summary>
+        /// Invoked on the server when a client connected.
+        /// </summary>
         ClientConnected,
+        /// <summary>
+        /// Invoked on all clients when a client connected.
+        /// </summary>
         PeerConnected,
+        /// <summary>
+        /// Invoked on all clients when a client disconnected.
+        /// </summary>
         ClientDisconnected,
+        /// <summary>
+        /// Invoked on all clients when a client disconnected.
+        /// </summary>
         PeerDisconnected
     }