Add explicit warning that custom embeds cannot be synchronized
Add explicit warning that custom embeds cannot be synchronized

bush6/sync,gv1222/sync,bush6/sync,6IRCNet/sync,6IRCNet/sync,bush6/sync,pajlada/sync,aykl/sync,Poniverse/Equestria.tv,6IRCNet/sync,gv1222/sync,nugget/sync,Poniverse/Equestria.tv,Poniverse/Equestria.tv,nugget/sync,gv1222/sync,nugget/sync,gv1222/sync,pajlada/sync,pajlada/sync,pajlada/sync,aykl/sync