Took a while, but HowTo is there now 
: HowTo: Setup Nextcloud Talk with TURN server
I made some clarification about the STUN and TURN addresses, which should be entered without any http(s):// or other protocol prefix, just the bare domain:port.
I did not intend to make a Talk usage guide. This would make it way too complex and this should be done as a separate HowTo or better documentation within Nextcloud Talks github wiki or docs.nextcloud.com e.g..
This should concentrate on why and how to setup the coturn correctly.