Add doc about server "flagged as disabled"

Problem/Motivation

I know it's bit out of the scope of this module, but I thinks the doc should explain how the memcache(d) client/lib cope with a dead server.
As I understand it, if a server is unreachable the lib "removed it from the list of servers".
(http://docs.libmemcached.org/memcached_behavior.html#MEMCACHED_BEHAVIOR_...)

I just made some tests with my devops (kill one of the three memcached server) and my Drupal website response time goes from under a second to more than six. I guess something is wrong somewhere in my setup (php.ini or settings.php), but the module doc doesn't explain what parameters should be set or checked.