Shouldn't we document "don't use a mountpoint as $PGDATA"?

I had a discussion with some folks at Red Hat about this:
https://bugzilla.redhat.com/show_bug.cgi?id=1247477

I had the idea that we had documented somewhere that the data directory
should not be a filesystem mount point, but I sure can't find it now.
Any objections to adding some text about this to section 17.2, near the
caveats about NFS?
        regards, tom lane