[doc] MathL::Petsc; Fix doxygen comments.

MathLib/LinAlg/PETSc/PETScMatrix.h

@@ -94,12 +94,12 @@ public:
     Mat const& getRawMatrix() const { return _A; }
     /// Set all entries to zero.
     void setZero() { MatZeroEntries(_A); }
-    /*
+    /*!
        \brief Set the specified rows to zero except diagonal entries, i.e.
               \f$A(k, j) = \begin{cases}
-                0.0, &j\not=k, j=1,2,\dots,k-1, k+1, \dots, n
+                0.0, &j\not=k, j=1,2,\dots,k-1, k+1, \dots, n \\
                 1.0, &j = k
-              \end{cases}f$, where \f$k \in \mbox{row\_pos}\f$
+              \end{cases}\f$, where \f$k \in \mbox{row\_pos}\f$
               This function must be called by all ranks.
        \param row_pos The row indicies of the specified rows.
     */
@@ -273,4 +273,4 @@ void PETScMatrix::add(std::vector<PetscInt> const& row_pos,
 bool finalizeMatrixAssembly(
     PETScMatrix& mat, const MatAssemblyType asm_type = MAT_FINAL_ASSEMBLY);
 
-}  // end namespace
+}  // namespace MathLib