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
Rename `geohashgrid` to `geohash_grid` in documentation.

It was renamed in fc6bc4c4776a2f710f57616e3495aaf6a230c4d3.

Close #4997
Does not support "script" in value_clunt aggregation.
[DOC] Tidied up geohashgrid aggregations
Added `cross_fields` mode to multi_match query

`cross_fields` attemps to treat fields with the same analysis
configuration as a single field and uses maximum score promotion or
combination of the scores based depending on the `use_dis_max` setting.
By default scores are combined.

Relates to #2959