Vacuum Cost Documentation?

Related to autovacuum work, I was looking into the new vacuum delay
functionality.  I might be missing something, but I can't find anything
on it in the developer docs.  Is that right?

Also, in the default postgresql.conf there is:

#vacuum_cost_naptime = 50       # 0-1000 milliseconds

however psql reports this value as 0.  I assume this is just
postgresql.conf is just wrong.  Can we fix this so it's consistent?

Thanks,

Matthew O'Connor