Issue #23530: Improve os.cpu_count() description.

Patch by Julian Taylor.

Doc/library/multiprocessing.rst

@@ -869,8 +869,13 @@ Miscellaneous
 
 .. function:: cpu_count()
 
-   Return the number of CPUs in the system.  May raise
-   :exc:`NotImplementedError`.
+   Return the number of CPUs in the system.
+
+   This number is not equivalent to the number of CPUs the current process can
+   use.  The number of usable CPUs can be obtained with
+   ``len(os.sched_getaffinity(0))``
+
+   May raise :exc:`NotImplementedError`.
 
    .. seealso::
       :func:`os.cpu_count`

Doc/library/os.rst

@@ -3596,6 +3596,11 @@ Miscellaneous System Information
 
    Return the number of CPUs in the system. Returns None if undetermined.
 
+   This number is not equivalent to the number of CPUs the current process can
+   use.  The number of usable CPUs can be obtained with
+   ``len(os.sched_getaffinity(0))``
+
+
    .. versionadded:: 3.4
 
 

Misc/ACKS

@@ -1385,6 +1385,7 @@ William Tanksley
 Christian Tanzer
 Steven Taschuk
 Amy Taylor
+Julian Taylor
 Monty Taylor
 Anatoly Techtonik
 Gustavo Temple

Modules/posixmodule.c

@@ -5760,7 +5760,7 @@ os.sched_getaffinity
     pid: pid_t
     /
 
-Return the affinity of the process identified by pid.
+Return the affinity of the process identified by pid (or the current process if zero).
 
 The affinity is returned as a set of CPU identifiers.
 [clinic start generated code]*/
@@ -11201,7 +11201,9 @@ get_terminal_size(PyObject *self, PyObject *args)
 /*[clinic input]
 os.cpu_count
 
-Return the number of CPUs in the system; return None if indeterminable.
+Return the number of CPUs in the system; return None if indeterminable.  This
+number is not equivalent to the number of CPUs the current process can use.
+The number of usable CPUs can be obtained with ``len(os.sched_getaffinity(0))``
 [clinic start generated code]*/
 
 static PyObject *