Add thread safety documentation

src/connection.cpp

@@ -5,6 +5,16 @@
 #include <errno.h>
 #include <limits.h>
 
+std::unique_ptr<Connection>
+Connection::createForServer(struct sockaddr_storage addr, int fd, int64_t id,
+                            ConnectionHandler *handler,
+                            std::weak_ptr<Server> server) {
+  // Use unique_ptr constructor with private access (friend function)
+  // We can't use make_unique here because constructor is private
+  return std::unique_ptr<Connection>(
+      new Connection(addr, fd, id, handler, server));
+}
+
 Connection::Connection(struct sockaddr_storage addr, int fd, int64_t id,
                        ConnectionHandler *handler, std::weak_ptr<Server> server)
     : fd_(fd), id_(id), addr_(addr), arena_(), handler_(handler),