[backport][math][Docs] Further improve GenVector documentation.

Changes similar to 23f2059

Author: devajithvs

math/experimental/genvectorx/inc/MathX/GenVectorX/LorentzVector.h

@@ -772,7 +772,7 @@ class LorentzVector {
 /**
    Scale of a LorentzVector with a scalar quantity a
    \param a  scalar quantity of type a
-   \param v  mathcore::LorentzVector based on any coordinate system
+   \param v  LorentzVector based on any coordinate system
    \return a new mathcoreLorentzVector q = v * a same type as v
  */
 template <class CoordSystem>

math/experimental/genvectorx/inc/MathX/GenVectorX/VectorUtil.h

@@ -44,6 +44,9 @@ namespace ROOT_MATH_ARCH {
 
 namespace VectorUtil {
 
+/// \addtogroup GenVector
+/// @{
+
 // methods for 3D vectors
 
 /**
@@ -259,6 +262,8 @@ inline typename Vector1::Scalar InvariantMass(const Vector1 &v1, const Vector2 &
    // return ( v1 + v2).mag();
 }
 
+/// @brief Returns the square of what InvariantMass(const Vector1&, const Vector2&) would return.
+/// This is mostly useful for speed optimisations, because the expensive sqrt operation is skipped.
 template <class Vector1, class Vector2>
 inline typename Vector1::Scalar InvariantMass2(const Vector1 &v1, const Vector2 &v2)
 {
@@ -561,6 +566,9 @@ double Phi_0_2pi(double phi);
  */
 double Phi_mpi_pi(double phi);
 
+// Close of doxygen group:
+/// @}
+
 } // namespace VectorUtil
 
 } // namespace ROOT_MATH_ARCH