Add explanation to verbosity in doc

In other words `cfg.verbose=0` could mean "I don't want to hear anything from CPP_BIDS", `cfg.verbose=1` could mean "I want to get my warnings", `cfg.verbose=2` could mean "tell me everything".

_Originally posted by @TomasLenc in https://github.com/cpp-lln-lab/CPP_BIDS/issues/150#issuecomment-738959736_