Improve documentation for argument factors in plot_factors()

[frederikziebell]

The documentation of plot_factors() says

a vector of length two with the factors to plot. Factors can be specified either as a characters.

I'd suggest to change it to

a vector of length greater than one with the factors to plot. Factors can be specified either as characters or numbers. If more than two factors are specified, a pairs plot with all pairwise factor combinations is shown.