add document to ClientHandshake about returning temporary error (#1125)

credentials/credentials.go

@@ -102,6 +102,10 @@ type TransportCredentials interface {
 	// authentication protocol on rawConn for clients. It returns the authenticated
 	// connection and the corresponding auth information about the connection.
 	// Implementations must use the provided context to implement timely cancellation.
+	// gRPC will try to reconnect if the error returned is a temporary error
+	// (io.EOF, context.DeadlineExceeded or err.Temporary() == true).
+	// If the returned error is a wrapper error, implementations should make sure that
+	// the error implements Temporary() to have the correct retry behaviors.
 	ClientHandshake(context.Context, string, net.Conn) (net.Conn, AuthInfo, error)
 	// ServerHandshake does the authentication handshake for servers. It returns
 	// the authenticated connection and the corresponding auth information about