Add warning note doc for nanotime
@@ -38,6 +38,11 @@ intrinsic fn instance() as system::instance;
## Returns the current time in epoch nanoseconds
##
+## WARNING: This function is expensive, please
+## avoid using it if you can use `ingest_ns` instead.
+## `ingest_ns` is virtually free as it is genreated
+## only once per event.
+##
## > ```tremor
## > use tremor::system;
## > let now = system::nanotime();
