Document usage of JSONEachrow reader

## Summary

Document briefly what is the idea and where to find examples
Define responsibilities
Note that feature is in beta

Also look into this comment:

JacksonJsonParserFactory builds a default ObjectMapper with no special number policy, while GsonJsonParserFactory uses ToNumberPolicy.BIG_DECIMAL so UInt64 and decimal JSON values are not materialized as Double. With JSON_DISABLE_NUMBER_QUOTING, Jackson-backed readers can lose precision or infer the wrong types for large integers and decimals.