SQL/JSON: Fix some obsolete comments.

SQL/JSON: Fix some obsolete comments.

JSON_OBJECT(), JSON_OBJETAGG(), JSON_ARRAY(), and JSON_ARRAYAGG()
added in 7081ac46ace are not transformed into direct calls to
user-defined functions as the comments claim. Fix by mentioning
instead that they are transformed into JsonConstructorExpr nodes,
which may call them, for example, for the *AGG() functions.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/058c856a-e090-ac42-ff00-ffe394f52a87%40gmail.com
Backpatch-through: 16