Adjust comments that called MultiXactIds "XMIDs".

Peter Geoghegan <pg@bowt.ie>

Commit: 9887dd38f9660721e4ef63333ab81a60b1b3da92
Author: Peter Geoghegan <pg@bowt.ie>
Date: 2022-08-30T02:42:30Z
Releases: 16.0
Adjust comments that called MultiXactIds "XMIDs".

Oversights in commits 0b018fab and f3c15cbe.

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