[Sketcher] [planegcs] Add references for equations

Some equations were mentioned by just the number without telling the resource they were from. The book was found couple years ago but not mentioned here.

src/Mod/Sketcher/App/planegcs/GCS.cpp

@@ -3712,6 +3712,8 @@ int System::solve(SubSystem *subsysA, SubSystem *subsysB, bool /*isFine*/, bool
             double alpha=1;
             alpha = std::min(alpha, subsysA->maxStep(plistAB,xdir));
 
+            // From the book "Numerical Optimization - Jorge Nocedal, Stephen J. Wright".
+            // See https://forum.freecadweb.org/viewtopic.php?f=10&t=35469.
             // Eq. 18.32
             // double mu = lambda.lpNorm<Eigen::Infinity>() + 0.01;
             // Eq. 18.33