Show various offset arrays for heap WAL records

Hi,

A couple times when investigating data corruption issues, the last time just
yesterday in [1], I needed to see the offsets affected by PRUNE and VACUUM
records. As that's probably not just me, I think we should make that change
in-tree.

The attached patch adds details to XLOG_HEAP2_PRUNE, XLOG_HEAP2_VACUUM,
XLOG_HEAP2_FREEZE_PAGE.

The biggest issue I have with the patch is that it's very hard to figure out
what punctuation to use where ;). The existing code is very inconsistent.

I chose to include infomask[2] for the different freeze plans mainly because
it looks odd to see different plans without a visible reason. But I'm not sure
that's the right choice.

Greetings,

Andres Freund

[1] https://postgr.es/m/CANtu0ojby3eBdMXfs4QmS%2BK1avBc7NcRq_Ot5bnzrbwM%2BuQ55w%40mail.gmail.com