Update execution.md

Author: AlexGuteniev

docs/standard-library/execution.md

@@ -29,12 +29,16 @@ namespace std::execution {
 |[parallel_policy Class](parallel-policy-class.md)|Used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm’s execution may be parallelized.|
 |[parallel_unsequenced_policy Class](parallel-unsequenced-policy-class.md)|Used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm’s execution may be parallelized and vectorized.|
 |[sequenced_policy Class](sequenced-policy-class.md)|Used as a unique type to disambiguate parallel algorithm overloading and require that a parallel algorithm’s execution may not be parallelized.|
+ 
+### Microsoft Specific
+ 
+When `parallel_policy` or `parallel_unsequenced_policy` cause the algorithm to be parallelized, the parallel execution uses Windows Thread Pool. See [Thread Pools](https://docs.microsoft.com/en-us/windows/win32/procthread/thread-pools). Number of concurrent threads is limited to thread pool defaults (currently 500). Number of threads concurrently executing on hardware is currently limited by number of logical processor in the process's processor group. See [Processor Groups](https://docs.microsoft.com/en-us/windows/win32/procthread/processor-groups). Number of chunks is also currently limited by number of logical processor in the process's processor group.
 
 ## Requirements
 
 **Header:** \<execution>
 
-**Namespace:** stdext
+**Namespace:** std
 
 ## See also