repl: document top level await limitation with const/let

Fixes: https://github.com/nodejs/node/issues/17669


Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/38449


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>