Add fullchain cert instruction comment to reverse proxy instructions to prevent android failure to connect.

[tyson-nw]

This issue respects the following points:

• This issue is not already reported on GitHub (I've searched it).
• I agree to follow Jellyfin's Code of Conduct.
• This report addresses only a single issue; If you encounter multiple issues, kindly create separate reports for each one.

Describe the missing documentation

A comment instructing the user to use the fullchain cert instead of the site cert in the nginx proxy instructions found on: https://jellyfin.org/docs/general/post-install/networking/reverse-proxy/nginx

This could also be included on the other reverse proxy instructions.

reasoning

It is the most common pitfall I have found preventing the android app from connecting to the server. A single line comment above the ssl_certificate directive along the lines of Ensure you are using the fullchain certificate not the site's certificate would prevent this from being an issue.

Other sources

jellyfin/jellyfin-android#742, https://www.reddit.com/r/jellyfin/comments/1pp15g2/android_app_cannot_connect_to_server_using/, jellyfin/jellyfin-android#742, jellyfin/jellyfin-android#1870, https://www.reddit.com/r/selfhosted/comments/1ewi0w6/issues_with_jellyfin_app_on_android/