I am reviewing these patches.

I notice that transam/README does not mention multixact except one place in section "Transaction Emulation during Recovery". I expected it to document how pg_multixact/members and pg_multixact/offset are used and what is their layout. It's not the responsibility of this patchset to document it, but it will be good if we add a section about multixacts in transam/README. It will make reviews easier.