Update future-functions.md

Author: AlexGuteniev

docs/standard-library/future-functions.md

@@ -58,6 +58,9 @@ In all cases, the associated asynchronous state of the `future` object is not se
 
 The pseudo-function `INVOKE` is defined in [\<functional>](../standard-library/functional.md).
 
+### Microsoft Specific
+When the passed function is executed asynchronously, it is executed on 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).
+
 ## <a name="future_category"></a> future_category
 
 Returns a reference to the [error_category](../standard-library/error-category-class.md) object that characterizes errors that are associated with `future` objects.