Update comments in multixact.c

Hi,

I noticed that commit 5212d447fa updated some comments in multixact.c because
SLRU truncation for multixacts is performed during VACUUM, instead of
checkpoint. Should the following comments which mentioned checkpointer be
changed, too?

1.
* we compute it (using nextMXact if none are valid).  Each backend is
* required not to attempt to access any SLRU data for MultiXactIds older
* than its own OldestVisibleMXactId[] setting; this is necessary because
* the checkpointer could truncate away such data at any instant.

2.
 * We set the OldestVisibleMXactId for a given transaction the first time
 * it's going to inspect any MultiXactId.  Once we have set this, we are
 * guaranteed that the checkpointer won't truncate off SLRU data for
 * MultiXactIds at or after our OldestVisibleMXactId.

Regards,
Shi yu