Clarify documentation of BiCubicInterpolator::Evaluate for out-of-bounds values

This updates outdated documentation and clarifies that supplying out-of-bound values for r and c is fine.
See https://github.com/ceres-solver/ceres-solver/issues/457.

Change-Id: I4a812786ebd05ec5bfedd388edbc71839434cc71

include/ceres/cubic_interpolation.h

diff --git a/include/ceres/cubic_interpolation.h b/include/ceres/cubic_interpolation.h
index 88a3ebf..a26697b 100644
--- a/include/ceres/cubic_interpolation.h
+++ b/include/ceres/cubic_interpolation.h
@@ -264,7 +264,8 @@
   }
 
   // Evaluate the interpolated function value and/or its
-  // derivative. Returns false if r or c is out of bounds.
+  // derivative. Uses the nearest point on the grid boundary if r or
+  // c is out of bounds.
   void Evaluate(double r, double c,
                 double* f, double* dfdr, double* dfdc) const {
     // BiCubic interpolation requires 16 values around the point being