bpo-32146: Add documentation about frozen executables on Unix (GH-5850)

Doc/library/multiprocessing.rst

@@ -186,6 +186,13 @@ A library which wants to use a particular start method should probably
 use :func:`get_context` to avoid interfering with the choice of the
 library user.
 
+.. warning::
+
+   The ``'spawn'`` and ``'forkserver'`` start methods cannot currently
+   be used with "frozen" executables (i.e., binaries produced by
+   packages like **PyInstaller** and **cx_Freeze**) on Unix.
+   The ``'fork'`` start method does work.
+
 
 Exchanging objects between processes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Misc/NEWS.d/next/Library/2018-02-25-10-17-23.bpo-32146.xOzUFW.rst

+Document the interaction between frozen executables and the spawn and
+forkserver start methods in multiprocessing.