Add documentation to deprecate WithTimeout dial option (grpc#1333)

clientconn.go

@@ -223,6 +223,7 @@ func WithPerRPCCredentials(creds credentials.PerRPCCredentials) DialOption {
 
 // WithTimeout returns a DialOption that configures a timeout for dialing a ClientConn
 // initially. This is valid if and only if WithBlock() is present.
+// Deprecated: use DialContext and context.WithTimeout instead.
 func WithTimeout(d time.Duration) DialOption {
 	return func(o *dialOptions) {
 		o.timeout = d