Clear differenciation of LFN, PFN, and DID in API documentation for declaring bad replica



When using the py client to declare a replica bad, there is a difference between using a LFN/DID and a PFN. LFN/DID will trigger the necromancer daemon, but using a PFN will trigger the minos daemon after it is added to the bad_pfns table. 

This difference should be included in documentation and a reason for picking either action. 

Reported by @yuyiguo 