pg_resetxlog -m documentation not up to date

The pg_resetxlog -m option was changed from
 -m XID           set next multitransaction ID

to
 -m XID,OLDEST    set next multitransaction ID and oldest value

but the man page does not reflect that change.

It was introduced in 0ac5ad5134f2769ccbaefec73844f8504c4d6182 "Improve
concurrency of foreign key locking", but there is also no explanation
there.  It is apparently needed in pg_upgrade.

This should be documented, and I think the help line should be changed
to something like
 -m XID,XID       set next and oldest multitransaction ID