refactor(validate-binding-behavior): document getTarget method
refactor(validate-binding-behavior): document getTarget method

fixes #257

aurelia/validation,ericIMT/validation,manuel-guilbault/aurelia-validation,manuel-guilbault/aurelia-validation,JeroenVinke/validation,aurelia/validation,JeroenVinke/validation,Zensavona/validation,ericIMT/validation