Update more obsolete multixact.c comments.
Update some remaining comments in multixact.c that still described SLRU
truncation as happening in the checkpointer, rather than during VACUUM.
Follow-up to commit
5212d447.
Shi yu, with tweaks by me.
Author: Shi yu <shiy.fnst@fujitsu.com>
Discussion: https://postgr.es/m/OSZPR01MB631066BF246F8F74E83222FCFDC69@OSZPR01MB6310.jpnprd01.prod.outlook.com