Index only scans and visibilitymap.c

With index only scans, the comments in
src/backend/access/heap/visibilitymap.c are probably out of date,
starting with:

"Currently, the visibility map is only used as a hint"

Also, is there a discussion of how and why index-only scans is safe?
i.e. what lock, if any, has to be held while nodeIndexonlyscan.c
calls visibilitymap_test? Do index-only scans need a README file?

Thanks,

Jeff