bfastpp: document external regressors better

There is lack of documentation in `?bfastpp` about how external regressors are handled. It is described, but an example of how to use them is missing.

Start with the ?bfastpp documentation and review the existing description of external regressors. Add a concrete example showing how to use external regressors, then confirm that the example accurately explains the documented behavior.

Written by the indexing model from the issue text.