Add information about relaying whole instances in admin-area

I tried to get relaying of a whole instance working. I tried pleroma.instance.tld https://pleroma.instance.tld

nothing worked. I got always "Server error" back...

Then, luckyly read in pleroma matrix room about the right url... its https://pleroma.domain.tld/relay

Please add this information in the administration-area as description where i can add a relay. And add what relaying a whole instance means.

AND

please add in code a little bit magic for parsing and trying to add /relay (and what is needed for mastodon for the same purpose), and give back a better error-message. For example something like this "Server gave back errorcode foo-bar. Is your url formed correct like https://instance.tld/relay "?