pythongh-105912: document gotcha with using os.fork on macOS

Using ``fork(2)`` on macOS when also using higher-level
system APIs in the parent proces can crash on macOS because
those system APIs are not written to handle this usage
pattern.

There's nothing we can do about this other than documenting
the problem.

Author: ronaldoussoren

Doc/library/os.rst

@@ -4373,6 +4373,11 @@ written in Python, such as a mail server's external command delivery program.
       If you use TLS sockets in an application calling ``fork()``, see
       the warning in the :mod:`ssl` documentation.
 
+   .. warning::
+
+      On macOS the use of this function is unsafe when mixed with using
+      higher-level system APIs, and that includes using :mod:`urllib.request`.
+
    .. versionchanged:: 3.8
       Calling ``fork()`` in a subinterpreter is no longer supported
       (:exc:`RuntimeError` is raised).
@@ -4412,6 +4417,11 @@ written in Python, such as a mail server's external command delivery program.
 
    .. audit-event:: os.forkpty "" os.forkpty
 
+   .. warning::
+
+      On macOS the use of this function is unsafe when mixed with using
+      higher-level system APIs, and that includes using :mod:`urllib.request`.
+
    .. versionchanged:: 3.12
       If Python is able to detect that your process has multiple
       threads, this now raises a :exc:`DeprecationWarning`. See the

Doc/library/pty.rst

@@ -33,6 +33,9 @@ The :mod:`pty` module defines the following functions:
    file descriptor connected to the child's controlling terminal (and also to the
    child's standard input and output).
 
+   .. warning:: On macOS the use of this function is unsafe when mixed with using
+      higher-level system APIs, and that includes using :mod:`urllib.request`.
+
 
 .. function:: openpty()
 

Doc/library/urllib.request.rst

@@ -21,6 +21,14 @@ authentication, redirections, cookies and more.
     The `Requests package <https://requests.readthedocs.io/en/master/>`_
     is recommended for a higher-level HTTP client interface.
 
+.. warning::
+
+   On macOS it is unsafe to use this module in programs using
+   :func:`os.fork` because the :func:`getproxies` implementation for
+   macOS uses a higher-level system API. Set the environement variable
+   ``no_proxy`` to ``*`` to avoid this problem 
+   (e.g. ``os.environ["no_proxy"] = "*"``).
+
 .. include:: ../includes/wasm-notavail.rst
 
 The :mod:`urllib.request` module defines the following functions: