doc for connecting remote repl from heroku through ssh tunnel

[seeeturtle]

https://devcenter.heroku.com/articles/exec#environment-variables
as you can see here, heroku doesn't apply environment variable when using ssh tunnel.
so if you set NREPL_PORT through heroku config it will be disabled and remote repl doesn't work.

i think it would be nice to have a guide about this. such as adding :nrepl-port config in prod.edn