Document that vacuum can't truncate if old_snapshot_threshold >= 0

Hi,

Currently, if old_snapshot_threshold is enabled, vacuum is prevented
from truncating tables:
static bool
should_attempt_truncation(LVRelStats *vacrelstats)
{
BlockNumber possibly_freeable;

possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
if (possibly_freeable > 0 &&
(possibly_freeable >= REL_TRUNCATE_MINIMUM ||
possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION) &&
old_snapshot_threshold < 0)
return true;
else
return false;
}

(note the old_snapshot_threshold < 0 condition).

That appears to not be mentioned in a comment, the commit message or the
the docs. I think this definitely needs to be prominently documented.

FWIW, afaics that's required because new pages don't have an LSN, so we
can't necessarily detect that a truncated and re-extended relation,
wouldn't be valid. Although I do wonder if there isn't a less invasive
way to do that.

Greetings,

Andres Freund