Clarify routerdesc expiration.

   failure count.

   Clients retain the most recent descriptor they have downloaded for each

   router so long as it is not too old (currently, 48 hours), OR so long as

   no better descriptor has been downloaded for the same router.

   [Versions of Tor before 0.1.2.3-alpha would discard descriptors simply for

   being published too far in the past.]  [The code seems to discard

   descriptors in all cases after they're 5 days old. True? -RD]

   router so long as it is listed in the consensus.  If it is not listed,

   they keep it so long as it is not too old (currently, ROUTER_MAX_AGE=48

   hours) and no better router descriptor has been downloaded for the same

   relay.  Caches retain descriptors until they are at least

   OLD_ROUTER_DESC_MAX_AGE=5 days old.

   Clients which chose to download the microdescriptor consensus instead

   of the general consensus must download the referenced microdescriptors