Add stability comment in doc

docs/nstrophy_doc/nstrophy_doc.tex

@@ -236,6 +236,12 @@ The enstrophy is defined as
   =\frac{4\pi^2}{L^2}\sum_{k\in\mathbb Z^2}k^2|\hat u_k|^2
   .
 \end{equation}
+\bigskip
+
+\point{\bf Numerical instability}.
+In order to prevent the algorithm from blowing up, it is necessary to impose the reality of $u(x)$ by hand, otherwise, truncation errors build up, and lead to divergences.
+It is sufficient to ensure that the convolution term $T(\hat\varphi,k)$ satifies $T(\hat\varphi,-k)=T(\hat\varphi,k)^*$.
+After imposing this condition, the algorithm no longer blows up, but it is still unstable (for instance, increasing $K_1$ or $K_2$ leads to very different results).
 
 \vfill
 \eject