Update 2.6.txt

Fixed typo: `.aggregation()` --> `.aggregate()`

Signed-off-by: kay <kay.kim@10gen.com>
'and' alone works fine here

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-2448: changing journaled write concern sentence
added a little more text on hidden mode as i fear people use it when they shouldnt
clarify hidden:true a tad
it is not necessary that latency is low, so removed that
Update isolated.txt