Improve pg_upgrade commentary on multixact change

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: cd7d00ad65fab11496ad9990cfc145c06c6ee53d
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2013-02-28T19:29:17Z
Releases: 9.3.0
Improve pg_upgrade commentary on multixact change

Per gripe from Andres Freund

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