message: Better documentation for PointMatrix
diff: @@ -767,10 +767,13 @@ protected: // implementation
  In 2D, the point matrix has the orientation of the parent
  edge, so its columns need to be flipped when applying it, see
  ApplyLocalSlaveTransformation.
  +
  In 3D, the orientation part of Elem2Inf is encoded in the point
  - matrix. The following transformation gives the relation betwen the
  - reference face coordinates (xi, eta) in [0,1]^2, and the fine face
  - coordinates (x, y):
  + matrix.
  +
  + The following transformation gives the relation betwen the
  + reference quad face coordinates (xi, eta) in [0,1]^2, and the fine quad
  + face coordinates (x, y):
  x = a0*(1-xi)*(1-eta) + a1*xi*(1-eta) + a2*xi*eta + a3*(1-xi)*eta
  y = b0*(1-xi)*(1-eta) + b1*xi*(1-eta) + b2*xi*eta + b3*(1-xi)*eta
  */
  