Added clarification to the help-message for the -B option.
Added clarification to the help-message for the -B option.

mittinatten/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,JoaoRodrigues/freesasa