godoc: add a note abount Shutdown state

We forgot to update a comment about the state.

Part of #257

Author: oleg-jukovec

connection.go

@@ -109,6 +109,11 @@ func (d defaultLogger) Report(event ConnLogKind, conn *Connection, v ...interfac
 // - In "Disconnected" state it rejects queries with ClientError{Code:
 // ErrConnectionNotReady}
 //
+// - In "Shutdown" state it rejects queries with ClientError{Code:
+// ErrConnectionShutdown}. The state indicates that a graceful shutdown
+// in progress. The connection waits for all active requests to
+// complete.
+//
 // - In "Closed" state it rejects queries with ClientError{Code:
 // ErrConnectionClosed}. Connection could become "Closed" when
 // Connection.Close() method called, or when Tarantool disconnected and