doc: Add information about shutdown_cb

terminates by calling spdk_app_stop() or an error condition occurs during the

initialization code within spdk_app_start(), itself, before invoking the caller's

supplied function.

## Custom shutdown callback {#event_component_shutdown}

When creating SPDK based application user may add custom shutdown callback which

will be called before the application framework starts the shutdown process.

To do that set shutdown_cb function callback in spdk_app_opts structure passed

to spdk_app_start(). Custom shutdown callback should call spdk_app_stop() before

returning to continue application shutdown process.