Add java documentation for aggregations

Buckets:
--------

* terms
* range
* global
* filter
* filters
* missing
* nested
* reverse nested
* children
* significant terms
* date range
* ip range
* range
* histogram
* date histogram
* geo distance
* geo hash grid

Metrics:
--------

* min
* max
* sum
* avg
* stats
* extended stats
* value count
* percentiles
* percentile rank
* cardinality
* geo bounds
* top hits
* scripted metric
Agreed.