Improve pg_search docs for query syntax on JSON columns

[neilyio]

What
We don't have enough examples of the query syntax for JSON objects. Some info from Tantivy on the query parser's syntax here:

https://github.com/quickwit-oss/tantivy/blob/main/doc/src/json.md

Example:

cart.product_type:sneakers AND cart.attributes.color:red

It's not clear to me whether there's really any way to query JSON objects with the advanced builders. If that's true, we should be explicit about that in the docs.