Document cells

[MasterMastic]

I'm trying to setup a listbox with a custom cell renderer and I'm completely lost. Documentation for seesaw.cells literally doesn't exist, yet it is referred to from listbox's docstring:

:renderer A cell renderer to use. See (seesaw.cells/to-cell-renderer).

I don't know Swing so it's hard for me to figure it out even when looking at the sources, but either way, cells need more love.