DOCS-4398: clarifies authenticationDatabase description

- --authenticationDatabase is required for mongotop and mongostat
  when connecting to a mongod or mongos that uses auth

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-3400: further clarifies default for non-tools DOCS-3400: clarifies default for mongo

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