Description of replace_force_set property is unclear

[tkuchida]

opensim-core/OpenSim/Simulation/Model/AbstractTool.cpp

Lines 212 to 216 in c39e912

	comment = "Replace the model's force set with sets specified in <force_set_files>? "
	"If false, the force set is appended to.";
	_replaceForceSetProp.setComment(comment);
	_replaceForceSetProp.setName("replace_force_set");
	_propertySet.append( &_replaceForceSetProp );

Appended to what?