net/rpc: document thread safety requirements of codec types.

Fixes #6306.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/13474043

src/pkg/net/rpc/client.go

@@ -58,6 +58,7 @@ type Client struct {
 // argument to force the body of the response to be read and then
 // discarded.
 type ClientCodec interface {
+	// WriteRequest must be safe for concurrent use by multiple goroutines.
 	WriteRequest(*Request, interface{}) error
 	ReadResponseHeader(*Response) error
 	ReadResponseBody(interface{}) error

src/pkg/net/rpc/server.go

@@ -616,6 +616,7 @@ func RegisterName(name string, rcvr interface{}) error {
 type ServerCodec interface {
 	ReadRequestHeader(*Request) error
 	ReadRequestBody(interface{}) error
+	// WriteResponse must be safe for concurrent use by multiple goroutines.
 	WriteResponse(*Response, interface{}) error
 
 	Close() error