Add a note about Trigg's correction

Add a pointer about how the theory and practice of Trigg's correction
for loss function differs when the second derivative of the loss
function becomes positive.

https://github.com/ceres-solver/ceres-solver/issues/573

Change-Id: Ic22ce91cc230f7ed7fa7b70a1cc2050919039828

docs/source/nnls_modeling.rst

@@ -1154,7 +1154,6 @@ the case, then its possible to re-weight the residual and the Jacobian
 matrix such that the robustified Gauss-Newton step corresponds to an
 ordinary linear least squares problem.
 
-
 Let :math:`\alpha` be a root of
 
 .. math:: \frac{1}{2}\alpha^2 - \alpha - \frac{\rho''}{\rho'}\|f(x)\|^2 = 0.
@@ -1178,6 +1177,11 @@ least squares algorithm to robustified non-linear least squares
 problems.
 
 
+While the theory described above is elegant, in practice we observe
+that using the Triggs correction when :math:`\rho'' > 0` leads to poor
+performance, so we upper bound it by zero. For more details see
+`corrector.cc <https://github.com/ceres-solver/ceres-solver/blob/master/internal/ceres/corrector.cc#L51>`_
+
 :class:`LocalParameterization`
 ==============================