[oneTBB] Clarify lightweight policy requirements (#407)

* Clarify lightweight policy requirements
* Improve wording for the lightweight policies

Signed-off-by: Fedotov, Aleksei <[email protected]>
Co-authored-by: Alexey Kukanov <[email protected]>

Author: aleksei-fedotov

source/elements/oneTBB/source/flow_graph/examples/lightweight_policy.cpp

@@ -8,10 +8,10 @@ int main() {
     function_node< int, int > add( g, unlimited, [](const int &v) {
         return v+1;
     } );
-    function_node< int, int, lightweight > multiply( g, unlimited, [](const int &v) {
+    function_node< int, int, lightweight > multiply( g, unlimited, [](const int &v) noexcept {
         return v*2;
     } );
-    function_node< int, int, lightweight > cube( g, unlimited, [](const int &v) {
+    function_node< int, int, lightweight > cube( g, unlimited, [](const int &v) noexcept {
         return v*v*v;
     } );
 

source/elements/oneTBB/source/flow_graph/functional_node_policies.rst

@@ -48,14 +48,18 @@ of a predecessor to handle this.
 Lightweight
 -----------
 
-This policy helps to reduce the overhead associated with the execution scheduling of the node.
+This policy allows to specify that the node body takes little time to process, as a non-binding hint
+for an implementation to reduce overheads associated with the node execution. Any optimization applied
+by an implementation must have no observable side effects on the node and graph execution.
 
-For functional nodes that have a default value for the ``Policy`` template parameter, specifying
-the ``lightweight`` policy results in extending the behavior of the default value of ``Policy``
-with the behavior defined by the ``lightweight`` policy. For example, if the default value of
+When combined with another policy, the ``lightweight`` policy results in extending the behavior
+of that other policy with the optimization hint. This rule automatically applies to functional nodes
+that have a default value for the ``Policy`` template parameter. For example, if the default value of
 ``Policy`` is ``queueing``, specifying ``lightweight`` as the ``Policy`` value is equivalent to
 specifying ``queueing_lightweight``.
 
+The function call ``operator()`` of a node body must be ``noexcept`` for lightweight policies to have effect.
+
 Example
 ~~~~~~~