Update include/deal.II/grid/grid_tools.h

Co-authored-by: Martin Kronbichler <martin.kronbichler@uni-a.de>

include/deal.II/grid/grid_tools.h

@@ -1110,8 +1110,12 @@ namespace GridTools
    *   account that the returned cell will only contain the point approximately.
    * @param[in] enforce_unique_mapping Enforce a one to one mapping between points
    *   in real and reference space.
-   * @param[in] marked_vertices Provide vertices that correspond to elements in
-   *   the reference points are searched
+   * @param[in] marked_vertices An array of bools indicating which
+   * vertices of @p mesh will be considered within the search
+   * as the potentially closest vertex. On receiving a non-empty
+   * @p marked_vertices, the function will
+   * only search among @p marked_vertices for the closest vertex,
+   * otherwise on all vertices in the mesh.
 
    * @return A tuple containing the quadrature information
    *