SPIGOT-4681: Note about additional world creation on STARTUP

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -874,6 +874,7 @@ public final class CraftServer implements Server {
 
     @Override
     public World createWorld(WorldCreator creator) {
+        Preconditions.checkState(!console.worldServer.isEmpty(), "Cannot create additional worlds on STARTUP");
         Validate.notNull(creator, "Creator may not be null");
 
         String name = creator.name();