Document the "Sybil" relay flag.

   published time on the descriptor is over 18 hours in the past.  (This flag

   was added in 0.4.0.1-alpha.)

   Directory server administrators may label some relays or IPs as

   blacklisted, and elect not to include them in their network-status lists.

   Authorities SHOULD 'disable' any relays in excess of 2 on any single

   IP.  When there are more than 2 (or AuthDirMaxServersPerAddr) to

   choose from, authorities should first prefer authorities to

   non-authorities, then prefer Running to non-Running, and then prefer

   high-bandwidth to low-bandwidth[*].  To 'disable' a relay, the

   authority *should* vote for it without the Running or Valid flag.

      [*] In this comparison, measured bandwidth is used unless it is not

          present for a router, in which case advertised bandwidth is used.

          Bug 8710 has a patch to change this behavior.

   "Sybil" -- authorities SHOULD NOT accept more than 2 relays on a single IP.

   If this happens, the authority *should* vote for the excess relays, but

   should omit the Running or Valid flags and instead should assign the "Sybil"

   flag.  When there are more than 2 (or AuthDirMaxServersPerAddr) relays to

   choose from, authorities should first prefer authorities to non-authorities,

   then prefer Running to non-Running, and then prefer high-bandwidth to

   low-bandwidth relays.  In this comparison, measured bandwidth is used unless

   it is not present for a router, in which case advertised bandwidth is used.

   Thus, the network-status vote includes all non-blacklisted,

   non-expired, non-superseded descriptors.