[MP] Fix server_relay being unsettable.

Clarify nuance in the docs.

modules/multiplayer/doc_classes/SceneMultiplayer.xml

@@ -79,6 +79,7 @@
 		</member>
 		<member name="server_relay" type="bool" setter="set_server_relay_enabled" getter="is_server_relay_enabled" default="true">
 			Enable or disable the server feature that notifies clients of other peers' connection/disconnection, and relays messages between them. When this option is [code]false[/code], clients won't be automatically notified of other peers and won't be able to send them packets through the server.
+			[b]Note:[/b] Changing this option while other peers are connected may lead to unexpected behaviors.
 			[b]Note:[/b] Support for this feature may depend on the current [MultiplayerPeer] configuration. See [method MultiplayerPeer.is_server_relay_supported].
 		</member>
 	</members>

modules/multiplayer/scene_multiplayer.cpp

@@ -610,7 +610,6 @@ Error SceneMultiplayer::object_configuration_remove(Object *p_obj, Variant p_con
 }
 
 void SceneMultiplayer::set_server_relay_enabled(bool p_enabled) {
-	ERR_FAIL_COND_MSG(multiplayer_peer.is_valid() && multiplayer_peer->get_connection_status() != MultiplayerPeer::CONNECTION_DISCONNECTED, "Cannot change the server relay option while the multiplayer peer is active.");
 	server_relay = p_enabled;
 }