Update obsolete multixact.c comments.

Update obsolete multixact.c comments.

Commit 4f627f89 switched SLRU truncation for multixacts back to being a
task performed during VACUUM, but missed some comments that continued to
reference truncation happening as part of checkpointing.  Update those
comments now.

Also update comments that became obsolete when commit c3ffa731 changed
the way that vacuum_multixact_freeze_min_age is applied by VACUUM as it
computes its MultiXactCutoff cutoff (which is used by VACUUM to decide
what to freeze).  Explain the same issues by referencing how OldestMxact
is the latest valid value that relminmxid can ever be advanced to at the
end of a VACUUM (following the work in commit 0b018fab).