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Add a note to the documentation about pre-built HLLSketches (#13088) 

* add a note to the documentation about pre-built HLLSketches

Druid actually supports ingesting a pre-generated sketch column by using
the HLLSketchMerge aggregator. However, this functionality was
previously not made clear in the documentation.

* copyedit from the King's English to American English

* add suggested style changes

Co-authored-by: Charles Smith <techdocsmith@gmail.com>

Co-authored-by: Charles Smith <techdocsmith@gmail.com>
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docs/development/extensions-core/datasketches-hll.md
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@ -59,6 +59,9 @@ druid.extensions.loadList=["druid-datasketches"]
}
```
The `HLLSketchBuild` aggregator builds an HLL sketch object from the specified input column. When used during ingestion, Druid stores pre-generated HLL sketch objects in the datasource instead of the raw data from the input column.
When applied at query time on an existing dimension, you can use the resulting column as an intermediate dimension by the [post-aggregators](#post-aggregators).
> It is very common to use `HLLSketchBuild` in combination with [rollup](../../ingestion/rollup.md) to create a [metric](../../ingestion/ingestion-spec.html#metricsspec) on high-cardinality columns. In this example, a metric called `userid_hll` is included in the `metricsSpec`. This will perform a HLL sketch on the `userid` field at ingestion time, allowing for highly-performant approximate `COUNT DISTINCT` query operations and improving roll-up ratios when `userid` is then left out of the `dimensionsSpec`.
>
> ```
@ -89,6 +92,8 @@ druid.extensions.loadList=["druid-datasketches"]
}
```
You can use the `HLLSketchMerge` aggregator to ingest pre-generated sketches from an input dataset. For example, you can set up a batch processing job to generate the sketches before sending the data to Druid. You must serialize the sketches in the input dataset to Base64-encoded bytes. Then, specify `HLLSketchMerge` for the input column in the native ingestion `metricsSpec`.
### Post Aggregators
#### Estimate