Extend the fail2ban instructions with a hint on how to make X-Real-IP… (#16446)

Following the merging of #14959 - Gitea is a lot more strict regarding the interpretation of `X-Real-IP` and `X-Forwarded-For` headers.

This PR updates the fail2ban documentation to include hints to set: `REVERSE_PROXY_TRUSTED_PROXIES` and `REVERSE_PROXY_LIMIT` appropriately.

See discussion in #16443

Co-authored-by: zeripath <art27@cantab.net>

docs/content/doc/usage/fail2ban-setup.en-us.md

@@ -108,3 +108,12 @@ this to your Nginx configuration so that IPs don't show up as 127.0.0.1:
 ```
 proxy_set_header X-Real-IP $remote_addr;
 ```
+
+The security options in `app.ini` need to be adjusted to allow the interpretation of the headers
+as well as the list of IP addresses and networks that describe trusted proxy servers
+(See the [configuration cheat sheet](https://docs.gitea.io/en-us/config-cheat-sheet/#security-security) for more information).
+
+```
+REVERSE_PROXY_LIMIT = 1
+REVERSE_PROXY_TRUSTED_PROXIES = 127.0.0.1/8 ; 172.17.0.0/16 for the docker default network
+```