DOC: Document default value for options.display.max_cols when not running in terminal
`display.max_cols` has a default value of 20 when not running in a terminal such as Jupyter Notebook.

This was a little confusing as the documentation for the default parameters generated in [Pandas User Guide - Available Options](https://pandas.pydata.org/docs/user_guide/options.html#available-options) are [environment specific](https://github.com/pandas-dev/pandas/blob/e493323d93e6a3447f0adfded493b80a42f4fec4/pandas/core/config_init.py#L412-L415) and 0 for when terminal is detected, but this was not noted in the current description.

Thanks @tmoschou 