documentation: improve level_ghost_owners

diff --git a/include/deal.II/distributed/tria_base.h b/include/deal.II/distributed/tria_base.h
index dbfe8d5cdf4a54c1ad67ee5d68428d69426c95d5..0e6507defc314aecad343ab9cf9f65be0ef248d7 100644 (file)
--- a/include/deal.II/distributed/tria_base.h
+++ b/include/deal.II/distributed/tria_base.h
@@ -179,8 +179,9 @@ namespace parallel
      * ghost cell adjacent to the cells of the local processor. In other
      * words, this is the set of subdomain_id() for all ghost cells.
      *
-     * @note If @p i is contained in the list of processor @p j, then @p j
-     * will also be contained in the list of processor @p i.
+     * The returned sets are symmetric, that is if @p i is contained in the
+     * list of processor @p j, then @p j will also be contained in the list of
+     * processor @p i.
      */
     const std::set<types::subdomain_id> &
     ghost_owners() const;
@@ -191,8 +192,14 @@ namespace parallel
      * other words, this is the set of level_subdomain_id() for all level
      * ghost cells.
      *
-     * @note If @p i is contained in the list of processor @p j, then @p j
-     * will also be contained in the list of processor @p i.
+     * The returned sets are symmetric, that is if @p i is contained in the
+     * list of processor @p j, then @p j will also be contained in the list of
+     * processor @p i.
+     *
+     * @note The level ghost owners can only be determined if the multigrid
+     * ownership has been assigned (by setting the
+     * construct_multigrid_hierarchy flag at construction time), otherwise the
+     * returned set will be empty.
      */
     const std::set<types::subdomain_id> &
     level_ghost_owners() const;