bpo-38778: Document that os.fork is not allowed in subinterpreters (GH-17123)

Small docs update for [bpo-34651](https://bugs.python.org/issue34651).

Other references to fork (e.g. the PyOS.*Fork functions or discussions of fork() when embedding Python) point back to os.fork, so I don't think any other updates are needed.

https://bugs.python.org/issue38778

Automerge-Triggered-By: @ericsnowcurrently
(cherry picked from commit b22030073b)

Co-authored-by: Phil Connell <pconnell@gmail.com>

Doc/library/os.rst

@@ -3475,7 +3475,11 @@ written in Python, such as a mail server's external command delivery program.
    parent.  If an error occurs :exc:`OSError` is raised.
 
    Note that some platforms including FreeBSD <= 6.3 and Cygwin have
-   known issues when using fork() from a thread.
+   known issues when using ``fork()`` from a thread.
+
+   .. versionchanged:: 3.8
+      Calling ``fork()`` in a subinterpreter is no longer supported
+      (:exc:`RuntimeError` is raised).
 
    .. warning::
 
@@ -3492,6 +3496,10 @@ written in Python, such as a mail server's external command delivery program.
    master end of the pseudo-terminal.  For a more portable approach, use the
    :mod:`pty` module.  If an error occurs :exc:`OSError` is raised.
 
+   .. versionchanged:: 3.8
+      Calling ``forkpty()`` in a subinterpreter is no longer supported
+      (:exc:`RuntimeError` is raised).
+
    .. availability:: some flavors of Unix.
 
 

Misc/NEWS.d/next/Documentation/2019-11-12-15-31-09.bpo-38778.PHhTlv.rst

@@ -0,0 +1 @@
+Document the fact that :exc:`RuntimeError` is raised if :meth:`os.fork` is called in a subinterpreter.