Add docs for element call guest spa.

Signed-off-by: Timo K <toger5@hotmail.de>

docs/config.md

@@ -344,6 +344,9 @@ The VoIP and Jitsi options are:
       this number is exceeded, the user will not be able to join a given call.
     - `brand`: Optional name for the app. Defaults to `Element Call`. This is
       used throughout the application in various strings/locations.
+    - `guest_spa_url`: Optional url for an Element Call SPA URL. If this is set Element Web will expose the option to share a join
+      link to public video rooms which can be shared to non matrix users.
+      The guest SPA typically is setup to use a homeserver that allows users to register without email ("passwordless guest users") and to federate.
 
 ## Bug reporting