Fix incorrect comment in JsonTablePlanJoinNextRow()

Fix incorrect comment in JsonTablePlanJoinNextRow()

The comment on the return-false path when both UNION siblings are
exhausted said "there are more rows," which is the opposite of what
the code does. The code itself is correct, returning false to signal
no more rows, but the misleading comment could tempt a reader into
"fixing" the return value, which would cause UNION plans to loop
indefinitely.

Back-patch to 17, where JSON_TABLE was introduced.

Author: Chuanwen Hu <463945512@qq.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/tencent_4CC6316F02DECA61ACCF22F933FEA5C12806@qq.com
Backpatch-through: 17