Clarify that JSON data type is still a string under the hood, at least in some contexts

[soerenwolfers]

https://github.com/duckdb/duckdb/discussions/11896

The documentation might benefit from a warning added to the current "The JSON extension makes use of the JSON logical type. The JSON logical type is interpreted as JSON, i.e., parsed, in JSON functions rather than interpreted as VARCHAR".

Page URL: https://duckdb.org/docs/extensions/json.html