bug6716 - document the aqbooksellers table

bug6716 - document the aqbooksellers table

This documents the table that stores vendor info in Koha.

There were 4 unused fields found:
  `accountnumber`
  `othersupplier`
  `currency`
  `booksellerurl`

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Accountnumber is no longer unused and can be managed.
(cherry picked from commit 378df7aea521435d63b35f6b6c7f8e166d6ff24f)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>
(cherry picked from commit fb1a9360c46d9aa33001fc6ea8985b4484a2c532)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>