Add documentation about testing the TURN server

In order to know if a TURN server is working it is useful to actually
test the data transfer through the TURN server instead of just checking
if it generates relay candidates.

When an RTCPeerConnection is created an RTCConfiguration can be provided
to configure it. One of the values it accepts is "iceTransportPolicy",
which can be either set to "all" to gather all types of candidates or to
"relay" to only gather relay candidates.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

docs/TURN.md

@@ -100,6 +100,18 @@ simple-log
 
 - The TURN server on `<yourChosenPortNumber>` needs to be accessible for all Talk participants, so you need to open it to the web and if your TURN server is running **behind a NAT**, forward it to the related machine. Also make sure to set the [`--external-ip` option](https://github.com/coturn/coturn/wiki/turnserver#options) when your TURN server is in a private network.
 
+#### 6. Testing the TURN server
+
+When the TURN server is set in the Talk settings a basic test against the TURN server is performed. You can perform a deeper test by forcing your browser to send the media of a call only through the TURN server:
+
+- Join a call
+- Open your browser console
+- Type `OCA.Talk.SimpleWebRTC.webrtc.config.peerConnectionConfig.iceTransportPolicy = 'relay'` in the console and press Enter
+- Leave the call
+- Join the call again
+
+Now, in that browser, the media sent to and received from other participants in the call should go through the TURN server. If the call works then the TURN server should work.
+
 ### What else
 Nextcloud Talk´s WebRTC handling is still mostly based on the one from the [Spreed.ME](https://www.spreed.me/) WebRTC solution. For this reason, all guides about how to configure coTURN for it, applies to Nextcloud Talk too.