Improve Aggregations documentation

* Mostly minor things like typos and grammar stuff
* Some clarifications
* The note on the deprecation was ambiguous. I've removed the problematic part so that it now definitely says it's deprecated
Fix confusing sentence

The original sentence didn't make much sense. I hope this is a bit better. Taken heavy inspiration from https://github.com/elasticsearch/elasticsearch/commit/c63d8c4fb566882bd4655ddc7dec13b4e8719244