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
In my dataset, height was a value between `1.5 and 2.3` randomly generated.
Remember what we have seen in scripted-metric agg:

```
"heights" : [ 1.122218480146643, -1.8148918111233887, -1.7626731575142909, ... ]
```

So I think a double is valid here.

Though I think I should may be do `if (x instanceof XXX)`? But it would complicate the example.

WDYT?