code: public final FacetOptions addFacetOnFlieldnames(Collection<String> fieldnames) {
  		Assert.notNull(fieldnames, "Fieldnames must not be null!");
  
  		for (String fieldname : fieldnames) {
  			addFacetOnField(fieldname);
  		}
  		return this;
  	}
docs: Append all fieldnames for faceting
  
  @param fieldnames
  @return