Add a note to the documentation.

diff --git a/include/deal.II/numerics/vector_tools.h b/include/deal.II/numerics/vector_tools.h
index 2fa2ba5250f0b6bcf93f0a5a5f12acda88847a97..f11f72611707a571e7b9b137bd9f1ddad82dc6eb 100644 (file)
--- a/include/deal.II/numerics/vector_tools.h
+++ b/include/deal.II/numerics/vector_tools.h
@@ -949,7 +949,9 @@ namespace VectorTools
    * be handled by this function and corresponding boundary value functions.
    * The keys of this map correspond to the number @p boundary_id of the face.
    * numbers::internal_face_boundary_id is an illegal value for this key since
-   * it is reserved for interior faces.
+   * it is reserved for interior faces. For an example of how to use this
+   * argument with a non-empty map, see the step-16 tutorial program. (Note
+   * that the argument type is equal to the FunctionMap type.)
    *
    * The flags in the last parameter, @p component_mask denote which
    * components of the finite element space shall be interpolated. If it is