docco on external storage?

Perhaps I am missing something, but the documentation on external 
storage appears remarkably sparse. It says it is better for substring 
searches, but otherwise doesn't say very much.

I have a table with two modest-length bytea fields (around 9K and 2K 
respectively) of already compressed data. I was looking for a clue as to 
whether or not these should use external storage - I suspect they should 
but I didn't see anything that gave me a definite answer.

I'd prepare doc updates on this if they were required but I don't have 
the requisite knowledge, nor time right now to go trawling endlessly to 
acquire it.

cheers

andrew