tb2rad_dir as config option is not documented

[TAlonglong]

Problem description

tb2rad_dir config option used here https://github.com/pytroll/pyspectral/blob/main/pyspectral/near_infrared_reflectance.py#L111 is not described in the config section eg. https://pyspectral.readthedocs.io/en/latest/installation.html#configuration-file

I figured out, but would be nice to have the documentation for future reference