Extended_stats now displays the upper and lower bounds on standard deviations (e.g. avg +/- std).
Default is to show 2 std above/below, but can be changed using the `sigma` parameter.
Accepts non-negative doubles
Closes#9356
* Make it clearer that `aggs` is an allowed synomym
for the `aggregations` key
* Fix broken example in for datehistogram, `1.5M` is
not an allowed interval
* Make use of colon before examples consistent
* Fix typos