add a comment to FractureIdentity2

MaterialLib/FractureModels/FractureIdentity2.h

@@ -16,7 +16,9 @@ namespace MaterialLib
 {
 namespace Fracture
 {
-
+/// Identity vector for extracting non-shear components of a fracture stress
+/// vector and a relative displacement vector, e.g. \f$\mathbf{m}=(0,1)^T\f$ in
+/// 2D case.
 template <int DisplacementDim>
 struct FractureIdentity2
 {