Update sharded-cluster-config-servers.txt

That statement is not true.  People following it has led to system failures due to config server overload.

Signed-off-by: Sam Kleinman <samk@10gen.com>
Cfg server update

Config servers can receive heavy load.  This statement is in many cases not correct.

Signed-off-by: Sam Kleinman <samk@10gen.com>