Document undocumented Server.voice_client property.

discord/server.py

@@ -88,6 +88,9 @@ class Server(Hashable):
         Indicates if the server is a 'large' server. A large server is defined as having
         more than ``large_threshold`` count members, which for this library is set to
         the maximum of 250.
+    voice_client: Optional[:class:`VoiceClient`]
+        The VoiceClient associated with this server. A shortcut for the
+        :meth:`Client.voice_client_in` call.
     mfa_level: int
         Indicates the server's two factor authorisation level. If this value is 0 then
         the server does not require 2FA for their administrative members. If the value is