Added --network flag example in parent command

Author: nikunj8866

src/Transient_Command.php

@@ -36,6 +36,10 @@
  *     # Delete all transients.
  *     $ wp transient delete --all
  *     Success: 14 transients deleted from the database.
+ * 
+ *     # Delete all site transients.
+ *     $ wp transient delete --all --network
+ *     Success: 2 transients deleted from the database.
  */
 class Transient_Command extends WP_CLI_Command {