A variety of small updates

1. Add answers to a number of FAQs.
2. Add a note to the documentation for NumericDiffCostFunction that
   NumericDiffOptions needs to be documented and mentioned.
3. Update the docs for Solver::Options::numeric_derivative_relative_step_size
   to indicate that this setting only applies to the gradient checker.
4. Remove deprecated constructors from NumericDiffCostFunction and
   DynamicNumericDiffCostFunction.

Change-Id: If8fc011b2a5996dbc2c51268aa477550ed014a1c

include/ceres/dynamic_numeric_diff_cost_function.h

diff --git a/include/ceres/dynamic_numeric_diff_cost_function.h b/include/ceres/dynamic_numeric_diff_cost_function.h
index c852d57..7f0a951 100644
--- a/include/ceres/dynamic_numeric_diff_cost_function.h
+++ b/include/ceres/dynamic_numeric_diff_cost_function.h
@@ -85,22 +85,6 @@
         options_(options) {
   }
 
-  // Deprecated. New users should avoid using this constructor. Instead, use the
-  // constructor with NumericDiffOptions.
-  DynamicNumericDiffCostFunction(
-      const CostFunctor* functor,
-      Ownership ownership,
-      double relative_step_size)
-      : functor_(functor),
-        ownership_(ownership),
-        options_() {
-    LOG(WARNING) << "This constructor is deprecated and will be removed in "
-                    "a future version. Please use the NumericDiffOptions "
-                    "constructor instead.";
-
-    options_.relative_step_size = relative_step_size;
-  }
-
   virtual ~DynamicNumericDiffCostFunction() {
     if (ownership_ != TAKE_OWNERSHIP) {
       functor_.release();

include/ceres/numeric_diff_cost_function.h

diff --git a/include/ceres/numeric_diff_cost_function.h b/include/ceres/numeric_diff_cost_function.h
index fa96078..5dfaeab 100644
--- a/include/ceres/numeric_diff_cost_function.h
+++ b/include/ceres/numeric_diff_cost_function.h
@@ -206,29 +206,6 @@
     }
   }
 
-  // Deprecated. New users should avoid using this constructor. Instead, use the
-  // constructor with NumericDiffOptions.
-  NumericDiffCostFunction(CostFunctor* functor,
-                          Ownership ownership,
-                          int num_residuals,
-                          const double relative_step_size)
-      :functor_(functor),
-       ownership_(ownership),
-       options_() {
-    LOG(WARNING) << "This constructor is deprecated and will be removed in "
-                    "a future version. Please use the NumericDiffOptions "
-                    "constructor instead.";
-
-    if (kNumResiduals == DYNAMIC) {
-      SizedCostFunction<kNumResiduals,
-                        N0, N1, N2, N3, N4,
-                        N5, N6, N7, N8, N9>
-          ::set_num_residuals(num_residuals);
-    }
-
-    options_.relative_step_size = relative_step_size;
-  }
-
   ~NumericDiffCostFunction() {
     if (ownership_ != TAKE_OWNERSHIP) {
       functor_.release();

include/ceres/solver.h

diff --git a/include/ceres/solver.h b/include/ceres/solver.h
index 318cf48..f53760e 100644
--- a/include/ceres/solver.h
+++ b/include/ceres/solver.h
@@ -701,10 +701,20 @@
     // this number, then the jacobian for that cost term is dumped.
     double gradient_check_relative_precision;
 
-    // Relative shift used for taking numeric derivatives. For finite
-    // differencing, each dimension is evaluated at slightly shifted
-    // values; for the case of central difference, this is what gets
-    // evaluated:
+    // WARNING: This option only applies to the to the numeric
+    // differentiation used for checking the user provided derivatives
+    // when when Solver::Options::check_gradients is true. If you are
+    // using NumericDiffCostFunction and are interested in changing
+    // the step size for numeric differentiation in your cost
+    // function, please have a look at
+    // include/ceres/numeric_diff_options.h.
+    //
+    // Relative shift used for taking numeric derivatives when
+    // Solver::Options::check_gradients is true.
+    //
+    // For finite differencing, each dimension is evaluated at
+    // slightly shifted values; for the case of central difference,
+    // this is what gets evaluated:
     //
     //   delta = numeric_derivative_relative_step_size;
     //   f_initial  = f(x)