SQL/JSON: Fix some obsolete comments.

JSON_OBJECT(), JSON_OBJETAGG(), JSON_ARRAY(), and JSON_ARRAYAGG()
added in 7081ac46ac 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

src/backend/parser/parse_expr.c

@@ -3709,11 +3709,9 @@ makeJsonConstructorExpr(ParseState *pstate, JsonConstructorType type,
 /*
  * Transform JSON_OBJECT() constructor.
  *
- * JSON_OBJECT() is transformed into json[b]_build_object[_ext]() call
- * depending on the output JSON format. The first two arguments of
- * json[b]_build_object_ext() are absent_on_null and check_unique.
- *
- * Then function call result is coerced to the target type.
+ * JSON_OBJECT() is transformed into a JsonConstructorExpr node of type
+ * JSCTOR_JSON_OBJECT.  The result is coerced to the target type given
+ * by ctor->output.
  */
 static Node *
 transformJsonObjectConstructor(ParseState *pstate, JsonObjectConstructor *ctor)
@@ -3903,10 +3901,11 @@ transformJsonAggConstructor(ParseState *pstate, JsonAggConstructor *agg_ctor,
 /*
  * Transform JSON_OBJECTAGG() aggregate function.
  *
- * JSON_OBJECTAGG() is transformed into
- * json[b]_objectagg[_unique][_strict](key, value) call depending on
- * the output JSON format.  Then the function call result is coerced to the
- * target output type.
+ * JSON_OBJECT() is transformed into a JsonConstructorExpr node of type
+ * JSCTOR_JSON_OBJECTAGG, which at runtime becomes a
+ * json[b]_object_agg[_unique][_strict](agg->arg->key, agg->arg->value) call
+ * depending on the output JSON format.  The result is coerced to the target
+ * type given by agg->constructor->output.
  */
 static Node *
 transformJsonObjectAgg(ParseState *pstate, JsonObjectAgg *agg)
@@ -3966,9 +3965,11 @@ transformJsonObjectAgg(ParseState *pstate, JsonObjectAgg *agg)
 /*
  * Transform JSON_ARRAYAGG() aggregate function.
  *
- * JSON_ARRAYAGG() is transformed into json[b]_agg[_strict]() call depending
- * on the output JSON format and absent_on_null.  Then the function call result
- * is coerced to the target output type.
+ * JSON_ARRAYAGG() is transformed into a JsonConstructorExpr node of type
+ * JSCTOR_JSON_ARRAYAGG, which at runtime becomes a
+ * json[b]_object_agg[_unique][_strict](agg->arg) call depending on the output
+ * JSON format.  The result is coerced to the target type given by
+ * agg->constructor->output.
  */
 static Node *
 transformJsonArrayAgg(ParseState *pstate, JsonArrayAgg *agg)
@@ -4004,11 +4005,9 @@ transformJsonArrayAgg(ParseState *pstate, JsonArrayAgg *agg)
 /*
  * Transform JSON_ARRAY() constructor.
  *
- * JSON_ARRAY() is transformed into json[b]_build_array[_ext]() call
- * depending on the output JSON format. The first argument of
- * json[b]_build_array_ext() is absent_on_null.
- *
- * Then function call result is coerced to the target type.
+ * JSON_ARRAY() is transformed into a JsonConstructorExpr node of type
+ * JSCTOR_JSON_ARRAY.  The result is coerced to the target type given
+ * by ctor->output.
  */
 static Node *
 transformJsonArrayConstructor(ParseState *pstate, JsonArrayConstructor *ctor)