Update Doc/library/subprocess.rst

Author: giampaolo

Doc/library/subprocess.rst

@@ -803,14 +803,24 @@ Instances of the :class:`Popen` class have the following methods:
 
    .. note::
 
-      When the ``timeout`` parameter is not ``None``, then (on POSIX) the
-      function is implemented using a busy loop (non-blocking call and short
-      sleeps). Use the :mod:`asyncio` module for an asynchronous wait: see
+      When ``timeout`` is not ``None``, an efficient event-driven mechanism
+      waits for process termination when available:
+
+      - Linux ≥5.3 uses :func:`os.pidfd_open` + :func:`select.poll`
+      - macOS and other BSD variants use :func:`select.kqueue`
+      - Windows uses ``WaitForSingleObject``
+
+      If none of these mechanisms are available, the function falls back to a
+      busy loop (non-blocking call and short sleeps). Use the :mod:`asyncio`
+      module for an asynchronous wait: see
       :class:`asyncio.create_subprocess_exec`.
 
    .. versionchanged:: 3.3
       *timeout* was added.
 
+   .. versionchanged:: 3.15
+      use efficient event-driven implementation on Linux >= 5.3 and macOS / BSD.
+
 .. method:: Popen.communicate(input=None, timeout=None)
 
    Interact with process: Send data to stdin.  Read data from stdout and stderr,