Add entry in Doc/whatsnew/3.15.rst

Author: giampaolo

Doc/whatsnew/3.15.rst

@@ -736,6 +736,20 @@ ssl
 
    (Contributed by Ron Frederick in :gh:`138252`.)
 
+subprocess
+----------
+
+* :meth:`subprocess.Popen.wait`: when ``timeout`` is not ``None``, an efficient
+  event-driven mechanism now waits for process termination, if available:
+
+  - Linux ≥5.3 uses :func:`os.pidfd_open` + :func:`select.poll`
+  - macOS and other BSD variants use :func:`select.kqueue` + ``KQ_FILTER_PROC``
+    + ``KQ_NOTE_EXIT``
+  - Windows keep using ``WaitForSingleObject`` (unchanged)
+
+  If none of these mechanisms are available, the function falls back to the
+  traditional busy loop (non-blocking call and short sleeps).
+  (Contributed by Giampaolo Rodola in :gh:`83069`).
 
 sys
 ---