Update OTP install docs to use certbot nginx plugin

docs/docs/installation/otp_redhat_en.md

@@ -196,8 +196,9 @@ sudo systemctl start nginx
 sudo certbot --email <your@emailaddress> -d <yourdomain> -d <media_domain> --nginx
 ```
 
-At this point if you open your (sub)domain in a browser you should see a 502 error, that's because Akkoma is not started yet.
+If that doesn't work the first time, add `--dry-run` to further attempts to avoid being ratelimited as you identify the issue, and do not remove it until the dry run succeeds. A common source of problems are nginx config syntax errors; this can be checked for by running `nginx -t`.
 
+If you're successful with obtaining the certificates, opening your (sub)domain in a browser will result in a 502 error, since Akkoma hasn't been started yet.
 
 ### Setting up a system service