Add some API docs on the nb_list minimizer option.
Specifically, I added doxygen comments to certain methods of MinMover
and MinimizerOptions. I wrote these comments because I just had to read
a bunch of code to figure out what nb_list did. I'm not an expert on
the minimizer, so these comments may not be perfectly complete or
accurate. But I don't think they're totally off-the-mark.