Document adding `customname`, `customcols`, and `customcode`.
Document adding `customname`, `customcols`, and `customcode`.

Had to do quite a bit of Googling and code archaeology to figure these out.
Hope they're correct. Corrections appreciated, especially for `customcode`,
which seems to be woefully underdocumented up to now.

ezekiel/bucardo,ezekiel/bucardo,ezekiel/bucardo