Add statements about [conf] to configurtation doc

We added a reference to it here:

https://docs.conan.io/en/latest/reference/config_files/global_conf.html#tools-configurations

But not here:

https://docs.conan.io/en/latest/configuration.html

https://github.com/conan-io/docs/blob/master/configuration.rst

Maybe we should, maybe not. Not sure.