net: document unimplemented methods and functions

Fixes #16802.

Change-Id: I41be7bb4e21e3beaa2136ee69771b0f455b2a7c6
Reviewed-on: https://go-review.googlesource.com/27417Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/file.go

@@ -6,6 +6,9 @@ package net
 
 import "os"
 
+// BUG(mikio): On NaCl and Windows, the FileConn, FileListener and
+// FilePacketConn functions are not implemented.
+
 type fileAddr string
 
 func (fileAddr) Network() string  { return "file+net" }

src/net/interface.go

@@ -10,6 +10,12 @@ import (
 	"time"
 )
 
+// BUG(mikio): On NaCl, Plan9 and Solaris, methods and functions
+// related to Interface are not implemented.
+
+// BUG(mikio): On DragonFly BSD, NetBSD and OpenBSD, the
+// MulticastAddrs method of Interface is not implemented.
+
 var (
 	errInvalidInterface         = errors.New("invalid network interface")
 	errInvalidInterfaceIndex    = errors.New("invalid network interface index")

src/net/iprawsock.go

@@ -9,6 +9,12 @@ import (
 	"syscall"
 )
 
+// BUG(mikio): On NaCl, Plan 9 and Windows, the ReadMsgIP and
+// WriteMsgIP methods of IPConn are not implemented.
+
+// BUG(mikio): On Windows, the File method of IPConn is not
+// implemented.
+
 // IPAddr represents the address of an IP end point.
 type IPAddr struct {
 	IP   IP

src/net/tcpsock.go

@@ -12,6 +12,9 @@ import (
 	"time"
 )
 
+// BUG(mikio): On Windows, the File method of TCPListener is not
+// implemented.
+
 // TCPAddr represents the address of a TCP end point.
 type TCPAddr struct {
 	IP   IP

src/net/udpsock.go

@@ -9,6 +9,15 @@ import (
 	"syscall"
 )
 
+// BUG(mikio): On NaCl, Plan 9 and Windows, the ReadMsgUDP and
+// WriteMsgUDP methods of UDPConn are not implemented.
+
+// BUG(mikio): On Windows, the File method of UDPConn is not
+// implemented.
+
+// BUG(mikio): On NaCl and Plan 9, the ListenMulticastUDP function is
+// not implemented.
+
 // UDPAddr represents the address of a UDP end point.
 type UDPAddr struct {
 	IP   IP