fix cross-refs and clarify overflow

[pdowler]

see ivoa-std/TAP#4

[msdemlei]

While I would still at least consider saying "if you can, indicate overflows in simple formats by setting the file name to something containing '-truncated'" or so – often, there is no streaming to the client involved and it is hence not hard to do that – I wont' belabour that point.

Of course +2 on fixing the baked-in references.

[mbtaylor]

My take on the overflow business is that file formats like CSV are just underpowered for that sort of thing - if you care about detecting overflows use a format that can do it, i.e. VOTable. If people want to (and can) smuggle messages into filenames good luck to them, but writing it into the standard puts something of a burden on clients to look for that behaviour.