Mention Jitsi support at the .well-known level in Jitsi docs

For https://github.com/matrix-org/matrix-react-sdk/pull/4348

docs/jitsi-dev.md

@@ -78,6 +78,9 @@ The Jitsi widget created by Riot uses a local `jitsi.html` wrapper (or one hoste
 `url`. The wrapper has some basic functionality for talking to Riot to ensure the
 required `postMessage` calls are fulfilled.
 
+**Note**: Per [jitsi.md](./jitsi.md) the `preferredDomain` can also come from the server's
+client .well-known data. 
+
 ## The Jitsi wrapper in Riot
 
 Whenever Riot sees a Jitsi widget, it ditches the `url` and instead replaces it with
@@ -92,3 +95,6 @@ call.
 Critically, when the widget URL is reconstructed it does *not* take into account the
 config.json's `preferredDomain` for Jitsi. If it did this, users would end up on different
 conference servers and therefore different calls entirely. 
+
+**Note**: Per [jitsi.md](./jitsi.md) the `preferredDomain` can also come from the server's
+client .well-known data.

docs/jitsi.md

@@ -39,6 +39,16 @@ should start a new conference on your Jitsi server.
 **Note**: The widget URL will point to a `jitsi.html` page hosted by Riot. The Jitsi
 domain will appear later in the URL as a configuration parameter.
 
+**Hint**: If you want everyone on your homeserver to use the same Jitsi server by
+default, set the following on your homeserver's `/.well-known/matrix/client` config:
+```json
+{
+  "im.vector.riot.jitsi": {
+    "preferredDomain": "your.jitsi.example.org"
+  }
+}
+```
+
 ## Mobile app support
 
 Currently the Riot mobile apps do not support custom Jitsi servers and will instead