Documentation update.

Update modeling.rst and solving.rst to reflect
changes to the API.

Change-Id: Id1a8adfed1486f08e5fd67c5af2d29708a26490c
diff --git a/include/ceres/solver.h b/include/ceres/solver.h
index 4fb6e4d..fc70073 100644
--- a/include/ceres/solver.h
+++ b/include/ceres/solver.h
@@ -365,7 +365,7 @@
 
     // Minimizer terminates when
     //
-    //   max_i |gradient_i| < gradient_tolerance * max_i|initial_gradient_i|
+    //   max_i |x - Project(Plus(x, -g(x))| < gradient_tolerance
     //
     // This value should typically be 1e-4 * function_tolerance.
     double gradient_tolerance;