Document a way to flow graph can be attached to arbitrary task_arena

doc/main/tbb_userguide/Flow_Graph_resource_tips.rst

@@ -14,4 +14,5 @@ in a flow graph.
 
    ../tbb_userguide/use_limiter_node
    ../tbb_userguide/use_concurrency_limits
-   ../tbb_userguide/create_token_based_system
+   ../tbb_userguide/create_token_based_system
+   ../tbb_userguide/attach_flow_graph_to_arena

doc/main/tbb_userguide/attach_flow_graph_to_arena.rst

@@ -0,0 +1,68 @@
+.. _attach_flow_graph_to_arena:
+
+Attach Flow Graph to an Arbitrary Task Arena
+======================
+
+
+|short_name| ``task_arena`` interface provides mechanisms to guide tasks execution within
+the arena by setting the preferred computation units or restricting part of computation units.
+In some case, you may want to use mechanisms within a flow graph.
+
+During its construction a ``graph`` object attaches to the arena, in which the constructing
+thread occupies a slot. Whenever a task is spawned on behalf of the graph, it is spawned
+in the arena of the graph it is attached to, disregarding the arena of the thread
+which is caused a task to be spawned.
+
+The example shows how to set the most performant core type as preferable for graph execution:
+
+
+::
+
+
+       std::vector<tbb::core_type_id> core_types = tbb::info::core_types();
+       tbb::task_arena arena(
+           tbb::task_arena::constraints{}.set_core_type(core_types.back())
+       );
+       arena.execute( [&]() {
+           graph g;
+           function_node< int > f( g, unlimited, []( int i ) {
+               /*the most performant core type is defined as preferred.*/
+           } );
+           f.try_put(1);
+           g.wait_for_all();
+       } );
+
+
+A ``graph`` object can be reattached to a different ``task_arena`` by calling
+the ``graph::reset()`` function. This reinitializes the ``graph`` and reattaches
+it to the task arena instance, inside which the ``graph::reset()`` method is executed.
+
+The example shows how reattach existing graph to an arena with the most performant core type
+as preferable for work execution:
+
+
+::
+
+
+       graph g;
+       function_node< int > f( g, unlimited, []( int i ) {
+           /*the most performant core type is defined as preferred.*/
+       } );
+       std::vector<tbb::core_type_id> core_types = tbb::info::core_types();
+       tbb::task_arena arena(
+           tbb::task_arena::constraints{}.set_core_type(core_types.back())
+       );
+       arena.execute( [&]() {
+           g.reset();
+       } );
+       f.try_put(1);
+       g.wait_for_all();
+
+See the following topics to learn more.
+
+.. toctree::
+   :maxdepth: 4
+
+   ../tbb_userguide/Guiding_Task_Scheduler_Execution
+   ../tbb_userguide/work_isolation
+