docs: improve documentation of OptimizeConn (#3910)

sys_conn.go

@@ -25,12 +25,16 @@ var _ OOBCapablePacketConn = &net.UDPConn{}
 
 // OptimizeConn takes a net.PacketConn and attempts to enable various optimizations that will improve QUIC performance:
 //  1. It enables the Don't Fragment (DF) bit on the IP header.
-//     This allows us to do DPLPMTUD (Path MTU Discovery).
+//     This is required to run DPLPMTUD (Path MTU Discovery, RFC 8899).
 //  2. It enables reading of the ECN bits from the IP header.
 //     This allows the remote node to speed up its loss detection and recovery.
 //  3. It uses batched syscalls (recvmmsg) to more efficiently receive packets from the socket.
+//  4. It uses Generic Segmentation Offload (GSO) to efficiently send batches of packets (on Linux).
 //
-// For this to work, the connection needs to implement the OOBCapablePacketConn interface (as a *net.UDPConn does).
+// In order for this to work, the connection needs to implement the OOBCapablePacketConn interface (as a *net.UDPConn does).
+//
+// It's only necessary to call this function explicitly if the application calls WriteTo
+// after passing the connection to the Transport.
 func OptimizeConn(c net.PacketConn) (net.PacketConn, error) {
 	return wrapConn(c)
 }