enhance documentation to cover robot search path and --robot-paths

@stdweird/@wpoely86/@geimer: please review (focus on the content, I'll make sure the references and formatting is correct)
it's replace, since both `--robot` and `robot-paths` have the action `store` (or `store_or_None`)

If a value for `robot-paths` is specified in both the configuration file and the environment, the latter wins and the cfgfile value will be ignored. That gives more control imho, without being a limited factor.