message: Add equations to explanation of the stopping criteria
diff: @@ -95,7 +95,8 @@ private:
  /**
  * The ResidualNormReduction class is a stopping criterion which stops the
  * iteration process when the residual norm is below a certain
  - * threshold relative to the norm of the initial residual.
  + * threshold relative to the norm of the initial residual, i.e. when
  + * norm(residual) / norm(initial_residual) < threshold.
  * For better performance, the checks are run thanks to kernels on
  * the executor where the algorithm is executed.
  *
  @@ -155,7 +156,8 @@ protected:
  /**
  * The RelativeResidualNorm class is a stopping criterion which stops the
  * iteration process when the residual norm is below a certain
  - * threshold relative to the norm of the right-hand side.
  + * threshold relative to the norm of the right-hand side, i.e. when
  + * norm(residual) / norm(right_hand_side) < threshold.
  * For better performance, the checks are run thanks to kernels on
  * the executor where the algorithm is executed.
  *
  @@ -213,7 +215,8 @@ protected:
  /**
  * The AbsoluteResidualNorm class is a stopping criterion which stops the
  * iteration process when the residual norm is below a certain
  - * threshold. For better performance, the checks are run thanks to kernels on
  + * threshold, i.e. when norm(residual) / threshold.
  + * For better performance, the checks are run thanks to kernels on
  * the executor where the algorithm is executed.
  *
  * @note To use this stopping criterion there are some dependencies. The
  