Add documentation about CostFunction::Evaluate.
Change-Id: I1df42c14f20c7c03f2a8ae21e75dda98cc592214
diff --git a/include/ceres/numeric_diff_cost_function.h b/include/ceres/numeric_diff_cost_function.h
index 4c84d4c..020d085 100644
--- a/include/ceres/numeric_diff_cost_function.h
+++ b/include/ceres/numeric_diff_cost_function.h
@@ -36,8 +36,10 @@
// To get an numerically differentiated cost function, you must define
// a class with a operator() (a functor) that computes the residuals.
//
-// The function must write the computed value in the last argument (the only
-// non-const one) and return true to indicate success.
+// The function must write the computed value in the last argument
+// (the only non-const one) and return true to indicate success.
+// Please see cost_function.h for details on how the return value
+// maybe used to impose simple constraints on the parameter block.
//
// For example, consider a scalar error e = k - x'y, where both x and y are
// two-dimensional column vector parameters, the prime sign indicates
