ipv4: add missing BUGS section

Change-Id: Idf0f3ce0cbd5eb402eb4929e4280b1bf746c04e9
Reviewed-on: https://go-review.googlesource.com/34432
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ipv4/doc.go

@@ -240,3 +240,5 @@
 // In the fallback case, ExcludeSourceSpecificGroup and
 // IncludeSourceSpecificGroup may return an error.
 package ipv4 // import "golang.org/x/net/ipv4"
+
+// BUG(mikio): This package is not implemented on NaCl and Plan 9.

ipv4/endpoint.go

@@ -12,6 +12,11 @@ import (
 	"golang.org/x/net/internal/netreflect"
 )
 
+// BUG(mikio): On Windows, the JoinSourceSpecificGroup,
+// LeaveSourceSpecificGroup, ExcludeSourceSpecificGroup and
+// IncludeSourceSpecificGroup methods of PacketConn and RawConn are
+// not implemented.
+
 // A Conn represents a network endpoint that uses the IPv4 transport.
 // It is used to control basic IP-level socket options such as TOS and
 // TTL.

ipv4/packet.go

@@ -9,6 +9,9 @@ import (
 	"syscall"
 )
 
+// BUG(mikio): On Windows, the ReadFrom and WriteTo methods of RawConn
+// are not implemented.
+
 // A packetHandler represents the IPv4 datagram handler.
 type packetHandler struct {
 	c *net.IPConn

ipv4/payload.go

@@ -6,6 +6,9 @@ package ipv4
 
 import "net"
 
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
 // A payloadHandler represents the IPv4 datagram payload handler.
 type payloadHandler struct {
 	net.PacketConn